Replenishment is a buff implemented in patch 3.0.2, triggered by some abilities and talents. It gives 10 party or raid members (with the lowest mana left) 1% of their maximum mana every 10 second for 15 seconds, or 1.5% over the full duration. Each time it is triggered, the 10 most needy members are recalculated, and those people either receive the buff, or have it refreshed on them.
Patch 4.0.1 (2010-10-12): Replenishment now grants .1% total mana every second over 15 seconds. Can now be triggered by Druids' [Revitalize]. [Hunting Party] no longer grants Replenishment.
Hotfix (2009-04-24):We are making a hotfix (any minute now probably) so that Replenishment in Arenas will only affect the character and not the whole team. It will continue to function as it does currently outside of Arenas.
We feel Replenishment is balanced for larger groups, but just makes too much of a difference on smaller Arena teams. Making the rules work differently in PvP and PvE isn't a solution we like to use often, but in this case we think it fixes the issue cleanly, while still making the talents in question useful to the character who takes them. Alternatives, such as nerfing the amount returned by Replenishment in Arena or making it not function at all, seem like more punitive choices from which we want to stay away.
Patch 3.1.0 (2009-04-14): Improved Water Elemental renamed to Enduring Winter, which gives Frost Bolt a 33/66/100% chance to grant Replenishment in addition to its' old effects. Improved Soul Leech gives spells which benefit from Soul Leech a 50/100% chance to proc Replenishment in addition to its' old effects.
Patch 3.0.3 (2008-11-04): Players below level 50 can now benefit from this effect.
[Enduring Winter] (Mage, Tier 4 Frost): Gives 
[Frostbolt] a 33/66/100% chance to grant Replenishment.
[Soul Leech] (Warlock, Tier 4 Destruction): Causes 
[Shadowburn], 
