Regeneration is a magical effect, found on variety of equipment, that increases In-Combat Health Regeneration Per-Tick of the caster by x, where x is the amount of extra hit points regained each tick.
This was one of the effects converted to a Blue Stat (we express it as
see Category: In-Combat_Health_Regeneration_(Equipment) for items with this effect.
Note: not all items with this type of effect were converted to blue stats, the few things that had a regen effect part of a bigger effect were not converted.
See Category: Regeneration for items with this effect.
This can also be shortened to "regen" and refer to the numerical value representing how fast your character regains health or power (in-combat or out-of-combat regen numbers both exist, and are viewable on the Stats tab of your Character page).