The overhead mark
Fixate is a mechanic that causes the mob to ignore its threat list and chase a specific player for a limited time.
Mobs introduced in Mists of Pandaria mark their current fixate target with an overhead mark resembling a pair of eyes.
The fixated player is supposed to run away and kite the mob around. Some fixate debuffs on players also give them a speed boost (Raigonn is an example).
Example mobs with fixate mechanics[]
Buru the Gorger (the first instance, not called a "Fixate")
Cinderweb Drone
Conqueror Krenna- Frozengore
- Gastropod
- Lilian's Soul (part of the Lilian Voss boss encounter)
- Molten Slag (heroic)
- Raigonn
- Rak'gor Bloodrazor
- Set'thik Windblade
- Spirit of the Lynx
The Pursuit ability of the 
Krick and Ick encounter is effectively a fixate effect, but it is not named such, and uses a different icon to highlight the fixated player.