Regen

Definition
The word "Regen" is an abbreviation of the word "Regeneration." Regen refers to the steady increase of a variable over time.

How does it apply to Guild Wars?
Regen can affect Health and Energy. Regen is set (for lack of a better word) on a player or MOB as an effect of a spell. For example, Life Siphon gives the caster Health Regen of X (X is dependant on the player's points in Blood Magic).