invincible is when you cannot die. some people think that invincibility is like a shield. others think that you can get cut up, you just cant actually DIE from it.
In halo 3 the invincible item can only be found in the campaign and when you use it you will become invincible to gun fire but you can be: assassinated or fall to your death
The only way you can become invincible in Mechquest it to level up to the top most level. You'll have so much life, not even a mod will be able to beat you.
For about 3 seconds in matchmaking after you collect the overshield, you are invincible. But, you can set custom games and forge games to make you invincilbe.