Mechanic

Mechanic in a video game is referring to a fixed course of action, which needs to be taken to properly advance in a specific game situation.

The term is mostly used for non-obvious (re)actions, which need to be learned in order to advance, but by definition includes any such action. E.g. having to block a skull attack is a very obvious game mechanic, while an example for a more complex mechanic would be avoiding the Cyberadication while fighting Brainiac in the Brainiac: The Command Ship raid:

Periodically Brainiac will teleport to the middle of the room at approximately 25/50/75% health and generate a shield, while preparing a powerful attack and having an orange icon above his head.
There will be three terminals in the room that players will need to activate quickly to remove the shield, and then a player will need to lunge Brainiac to counter him.
If Brainiac isn't interrupted in time (approximately 10 seconds from when his shield goes up) he will unleash a powerful shockwave (Cyberadication) to the group. In the normal version of the raid this will deal large damage, however in the elite version this will likely cause a group wipe.

Especially raids may feature more complex mechanics. However many of those are hinted by on-screen texts or audio tells, so players would be able to prepare and react properly if paying attention.

In elite and elite plus instances, failing mechanics is almost always lethal. On the other hand instances can be finished even by average players by properly following the required mechanics.