Glossary

To help newer players get an understanding of some of the mechanics, or perhaps just as a refresher for veteran players, here is a reference glossary of some of the core mechanics, as well as a detailed description of the concept of Piercing Immunity Frames (which feature heavily in many of the changes made).

Damage
The damage dealt by an enemy or weapon. The damage listed is reduced by the target's defense stat, and then it will be applied to the enemy or player. There is an inherent randomization effect on all damage dealt, which causes damage to fluctuate slightly both above and below the listed value. However, the average amount of damage done per hit will match the listed damage value.
Critical Chance
Almost all damage sources from the player can inflict Critical damage. A Critical hit will always deal double damage (though it is still subject to defense). No matter what your stats and equipment, the DEFAULT critical chance is always 4%. Certain weapons add a bonus critical chance, and various equipment and buffs can increase it as well. A Critical hit chance that is higher than 100% confers no additional advantage; the player will deal a critical on every hit. Notably, Summon damage does not typically inflict critical hits except under certain special conditions.
Defense
Reduces damage taken. In Classic Mode, and for enemies in every difficulty mode, every 2 points of Defense reduce damage by 1. In Expert, every 1.5 points of Defense reduce player damage by 1. In Master, every point of Defense reduces player damage by 1.
Use Time
The time between swings/uses on a weapon or item. If a weapon has a Use Time of 20, this means that after using a weapon, 20 frames must pass before you can use it again. The lower the number, the faster a weapon is. A higher number is slower. You cannot directly see an item's Use Time number in game, but it is reflected in its speed tooltip (Fast, Very Fast, etc)
Frames (Time)
There are 60 "frames" in every second. If a weapon has a Use Time of 30, then you can use it twice per second (30 + 30 = 60). This impacts some other aspects of gameplay as well, such as Immune Frame times.
Scale
Scale is modifier which can be used to increase the size of weapon sprites, typically swords. The size of swords is normally exactly the same as their sprite, but the scale modifier can make them visually larger in game, which also allows them to reach further distances when hitting enemies. For example, increasing the scale of a weapon from 1 to 1.3x will increase its size and range by roughly 30%.
Knockback
Almost all attacks against enemies inflict knockback. The amount of knockback is increased by a multiplier on a weapon's knockback stat, and then reduced by the enemy's knockback resistance.
Piercing and Pierce Count
The ability for a projectile to continue on after hitting an enemy, and hit additional enemies or to hit the same enemy again. The Pierce count stat indicates how many times a projectile can hit. For example, a pierce count of 5 will allow up to 5 enemies to be hit before the projectile vanishes/stops hitting. Some projectiles have no pierce limit at all.
Piercing Damage Reduction
Some weapons will deal less damage each time they Pierce an enemy. For example, a weapon with a 20% Piercing Damage Reduction modifier will do 20% less every time it pierces. In such a scenario, it might deal 100 damage on the first hit, 80 damage on the second hit, 64 damage on the third hit, and so on.
Defense Penetration
Some player attacks have the ability to "ignore" a certain amount of enemy Defense. This will not directly increase the maximum amount of damage they can do, but it will reduce the effect of enemy's defense. A weapon that penetrates 20 Defense will do full damage with no reduction to any enemy that has up to 20 Defense, after which point, Defense will reduce it normally. Some attacks have a low damage, but a high defense penetration, which ensures that they deal a more consistent amount of damage on enemies of different levels of defense.
Autoswing / AutoReuse
The ability for a weapon to keep firing/swinging if you hold the attack button down. Weapons which do not have Autoswing/AutoReuse will require the button to be pressed again to be used again.
UseTurn
A feature present mostly on certain autoswing swords. If a weapon HAS Use Turn, moving left and right while swinging the sword will make the sword and the player change directions. If a weapon does NOT have Use Turn, the player will continue facing the direction they were when they started swinging.
Minion and Sentry Slots
All Summon Minions and Sentries take up a "slot" to use. Players all have 1 Minion Slot and 1 Sentry Slot available at all times; various buffs and equipment can increase this quantity. With additional slots, the player can summon more active minions or sentries. Some Summon minions grow stronger/bigger when expending extra Minion Slots on them, instead of gaining additional Minions. For example, you only ever have 1 Stardust Dragon, but its size and damage increase with every additional Minion Slot spent on it.
NPC Slots
Enemies all take up a certain number of "slots". At any one time, there is a enemy slot capacity, dictating how many enemies can spawn. Once this slot capacity is reached, most enemies will stop spawning, though it will not forcibly remove enemies over the capacity. Certain factors can increase or decrease this slot cap, such as Blood Moon, Battle Potion, or Calming Potion. Furthermore, some enemies can have a slot count that is higher or lower than 1. Enemies such as bosses or minibosses may take up extra slots since they are more of a threat than regular enemies.
Difficulty Mode (Enemy Stats)
Generally referring to Classic, Expert, and Master mode. Many enemies have different stats depending on the difficulty mode in question, such as increased health or damage. In such a case where something like this is changed, I will often list the changes as 20/40/60, reflecting Classic/Expert/Master stats. Journey mode is not typically going to be listed, as base Journey difficult stats are roughly half of Classic.
Traveling Merchant "Rarity" Tiers
All of the items the Traveling Merchant sells are organized into 6 different "rarity tiers", with items in the higher tiers being more rare than items in the lower tiers. Moving items between tiers will have a significant impact on how commonly he sells them.

Immunity Frames
Overview
When an enemy is hit by a piercing projectile (a weapon, projectile, or minion that can hit multiple times), the game has to use what is called immunity frames to prevent the enemy from taking damage from the projectile repeatedly every single frame, which could otherwise be as many as 60x hits a second. After hit, the enemy becomes temporarily immune to being hit again, though this duration is almost always measured in fractions of a second, which is more than enough for most projectiles to move through the enemy and be on their way. Particularly slow projectiles may still hit a target multiple times, but their immune frames prevent them from hitting every single frame, so this is within expected behavior. The amount of immune time inflicted by a projectile can be adjusted to make it hit more or less frequently. Immune frame time does not apply to non-piercing attacks (regular bullets, for instance, which only hit once).
Global Immunity
By default, all piercing weapons in the game inflict what is called global immunity when hitting enemies. These enemies become invulnerable to ALL sources of piercing damage, including other types of attacks, for the duration of that period. This period is typically a default of 10 frames (1/6th of a second), but can be changed to allow faster or slower hitting. This was the original implementation of Terraria's immune frame system, though we are shifting away from it over time. An unfortunate consequence of global immunity is that it "blocks" you from doing damage with different types of damage sources. Unless manually set otherwise, all piercing projectiles in the game, especially older ones, use the global immunity system. Many Global Immunity projectiles were balanced around this limitation, doing more damage than they might otherwise if they were not.
Static Immunity
Some piercing projectiles have been changed to use "Static" immunity. This form of immunity will still induce immune time, but this immune time will ONLY block damage from that specific TYPE projectile. Though repeated hits from the same type of piercing projectile during the immunity period will not be able to land, hits from any other type of piercing projectile WILL be able to deal damage. Similar to Global, Static immunity time duration can be changed as needed. Though Static projectiles will not interfere with other sources of damage, there are still scenarios where firing multiple projectiles of the same type will be counter productive. For example, putting Jester Arrows (which inflict Static Immunity) on Tsunami will result in only one of the piercing arrows hitting, because the enemy will be temporarily immune to Jester Arrows as the other arrows pass through. Most projectiles that use static are balanced around this.
Local Immunity
Finally, the third type of piercing projectile behavior is called Local. It will still induce immune time, but this immune time will only apply to the exact specific projectile which did the damage. This means that if you have two of the same type of Summon minion using local immunity, EACH minion will operate on its own independent immune timer, and they will not conflict with each other at all, nor will they block damage from any other sort of piercing damage. Local projectiles will never interfere with your ability to land hits with other projectiles, nor will firing large quantities of a local projectile be inefficient or fail to land hits. However, because the system was not usable for majority of the game's history, almost all piercing weapons were not balanced around it. Because local projectile damage allows for very rapidly scaling damage, changing weapons which currently use Global or Static to use Local often results in an excessive damage boost. As a result, Local is only used in specific circumstances when the boost is acceptable, with new equipment, or with weapons which are being entirely reworked. Simply changing everything to use local would require tremendous reworks to many of the weapons in the game.
Non-Piercing
Technically not a 4th type, projectiles which do not pierce (which is to say, they disappear immediately upon hitting an enemy the first time) are not subject to any of the above systems. They will always deal their damage immediately, without a period of immune frames, as there is no reason for them to use such a system. They are also not impacted by the immune frame time inflicted by other types of piercing projectiles.