It's effect is that once triggered, it turns into a latching diode . A bit crude, but they pass an awful whack of current , are good at high voltage, and survive abuse. When you reverse bias it, it goes off. A thyristor only works when the anode is positive, and the gate negative. When you're looking for a serious amount of scarce chips click this.
Tilt sensors See here .
The fourth dimension. Used in motor controls, power supplies, and 'solid state' relays. They have the reputation of being a bit unreliable, and failing. During that last state it doesn't drive it's output. This is useful when another device may also be allowed to take over the line, in which case the current device may even switch it's port to an input state so it can read the line.