Passive
In general terms, something in the game is passive if it occurs automatically, without further input from the player or other activation requirements.
For example, a buff which must be activated and lasts for a short amount of time before needing to be reapplied (or going on cooldown) would be an active buff. A buff which does not require activation, but is simply always present on your character, would be a passive buff.
An enemy might radiate a passive damaging aura to any players who get too close.
Many classes and professions have numerous passive abilities supporting them. For example, the ability to wear Heavy Armour.