Online:Synergy

Synergies are special abilities that other allied players may activate when you use certain skills.  When a nearby player is using a skill with a Synergy option, you will see a prompt at the bottom of the screen letting you know that you can activate the Synergy, by pressing the key or buttons associated with it.  ("X" by default on PC/Mac, 20px|Y+20px|B on Xbox, 20px|Triangle+20px|Circle on PS4).  You may do so at no cost to yourself, though some Synergies may cause the effects to expire sooner.  Generally, only one player may activate a Synergy on a given skill, though there are exceptions.  The Undaunted skill line is particularly based on Synergy, as all of its active skills have one associated with them.  Some Synergies only appear with certain skill morphs, or change depending on the morph chosen.

Cooldown
Each synergy has a 20 second cooldown before that same synergy can be used again by the same player. A single player can use several different synergies in quick succession, but each player must wait 20 seconds before using the same class of synergy again.

Luminous Shards and Energy Orb's synergies uniquely share the same cooldown. Using one will prevent you from using the other for 20 seconds. These are the only two synergies that share a cooldown.

Mechanics
When in range of the synergy, which is usually the spell's area of affect, anyone in the group other than the player casting it can activate the synergy. After that, that synergy is no longer available to anyone for 20 seconds. However, several players can use the same synergy in the same second (but this may be a visual bug). Several synergies can overlap, and are treated as a first-come-first-serve basis. 

A few synergies have conditions and exceptions, such as the Purify synergy from Cleansing Ritual that can be used by up to 3 players (unverified), or the Blood Funnel synergy from Blood Altar only appearing when the player has a certain percentage of health missing.


List of Synergies
Undaunted Skills
 The Blood Altar ability and its morphs provide the   Synergy, which allows a player to restore health.  The Overflowing Altar morph changes it to  , which increases the health restored.
  The Trapping Webs ability provides the   Synergy, which deals Poison Damage and summons spiders to attack your enemies.  Its morphs provide different versions of the Synergy.  Specifically, Shadow Silk changes it to  , whose summons last longer and do more damage.  Tangling Webs changes it to  , which also causes fear to your enemies.
The Inner Fire ability and its morphs provide the   Synergy, which deals Magic Damage to your targets.  The Inner Rage morph increases the chance of a Synergy occurring.
 The Bone Shield ability provides the   Synergy, which grants players a shield which absorbs all incoming damage up to 60% of their maximum health.  The Bone Surge morph changes this to  , which additionally increases their healing received.
  The Necrotic Orb ability provides the   Synergy, which causes the orb to explode and deal Magic Damage and restore the player's Magicka or Stamina, whichever source has the highest maximum amount. Energy Orb changes it to   which causes the activating player to also be healed.

Dragonknight Skills
The Dragonknight Standard Ultimate ability and its morphs provide the   Synergy, which immobilizes all enemies within range and deals Fire Damage.
The Dark Talons ability and its morphs grant the   Synergy, which deals Magic Damage to trapped enemies.

Necromancer Skills
The Boneyard ability allows players activate the   Synergy, which deals additional Frost Damage to enemies in the area, and also be healed for the amount of damage done.  Uniquely, the Avid Boneyard morph allows the Necromancer to activate their own synergy.
The Agony Totem morph of Bone Totem grants players the   Synergy, dealing Magic Damage to affected enemies and afflicting them with Minor Vulnerability, increasing their damage taken.

Nightblade Skills
The Consuming Darkness Ultimate ability grants players whose health is below a certain threshold.  the   (formerly "Slip Away") morph, which restores their health, increases their movement speed, and grants them temporary invisibility.
The Soul Shred Ultimate ability and its morphs grant the   Synergy, which allows players to steal health from an affected enemy.

Sorcerer Skills
The Lightning Splash ability and its morphs grant the   Synergy, which does additional damage to the enemies.
The Summon Storm Atronach Ultimate ability and its morphs grant the   Synergy, which increases both their damage and the atronach's.

Templar Skills
 The Spear Shards ability grants the   Synergy, which restores a player's   or  , whichever source has the highest maximum amount.  The Luminous Shards morph changes it to  , which also restores a small amount of the other attribute.
 The Nova Ultimate ability grants the   Synergy, which deals Magic Damage and stuns affected enemies in the area for a few seconds.  The Solar Prison morph changes this to  , which lasts longer and deals more damage.
The Cleansing Ritual ability and its morphs grant the   Synergy, which removes harmful effects and provides additional healing.

Warden Skills
 The Healing Seed ability and its morphs grant the   Synergy, which restores the player's   immediately.
 The Frozen Retreat morph of Frozen Gate grants the   Synergy, which allows a player to transport instantly to your location, useful if you are playing a healer.

Werewolf Skills
The Howl of Despair morph of Piercing Howl grants the   Synergy, which increases the attack damage of your allies.

Synergy-Like Events
 
Sometimes, there are other events which behave like Synergies, giving you the option to perform an action during combat.  They are activated in the same way, but do not require another player to be around, or trigger any abilities which are affected by Synergies.  Some examples are:

The Feed ability shows when you are a Vampire and sneaking close enough to a humanoid target to feed.  Doing so will reduce your Vampirism Stage by one, as well as restoring your Health.  Your Vampirism stage will increase every 6 hours if you do not feed. Note that this may be preferable, as some of the Vampire abilities are more powerful if you are in higher Stages of Vampirism. Doing so is considered an attack, and will be reported as a crime if done to a non-hostile enemy. The range is slightly higher than the Blade of Woe ability.  The same ability shows when giving another player a bite at a Vampire Shrine with the Blood Ritual skill, though this does not reduce your Vampirism stage.
The Devour ability can be used by Werewolves provided they have invested in the Devour skill line and are currently in Werewolf form.  This allows you to consume the corpses of your slain enemies and remain in Werewolf form longer, as well as restoring your Health.  The same ability shows when giving another player a bite at a Werewolf Shrine with the Bloodmoon skill.
During the final battle in Arx Corinium against Sellistrix the Lamia Queen, the Nereid sisters will give you the option to "Taunt Her!" (If you're not playing the tank role, it's best to avoid using this.)
During Stone Cold, you have to Use Indanas to Collapse Hole.
During the quest God of Schemes, there are two instances where you are given a pseudo-Synergy event, first to form floating rocks into a bridge using Aedric Command, and later to escape from Molag Bal's clenched fist using Sacrifice of Courage.
In Imperial City Prison , during the battle with Ibomez the Flesh Sculptor, he will drop flesh grenades, which can be activated like a Synergy to deal Disease Damage to the enemies in the room.
If you rescue Unsulag during the Forcing the Faith  quest in Paragon's Remembrance, she will cast a healing spell which gives you a Synergy called Restoring Waves which restores your health.
Sigils are a type of synergy available in certain Arenas by activating the glowing token during combat.
The Blade of Woe  ability can be used by sneaking close enough to a humanoid target as long as you have the relevant Dark Brotherhood passive skill. It instantly kills weak humanoids and is therefore useful for murdering Justice Neutral NPCs. The range is slightly lower than a vampire's Feed ability.
In the Cradle of Shadows , there are Flesh Atronachs with torches which can be looted from them with a Synergy when they are killed.  Doing so will protect you and your group from the darkness in the later sections.
At Shipwreck Cove  on Vvardenfell, Kimbrudhil the Songbird has an attack which will Levitate all nearby players, and most likely kill them.  The only reliable way to escape this is with a Synergy called Escape her Embrace.