Circle of Healing is a Holy priest talent. It is an instant-cast spell that heals up to 5 nearby party or raid members within a 30 yard radius of the target. It does not necessarily heal the person you cast the spell on.
Without improvements, Circle of Healing is 5% more efficient than Prayer of Healing, therefore eclipsing it in terms of speed, HPS and HPM. In these cases Circle of Healing should be used instead of Prayer of Healing whenever possible.
Circle of Healing is a major contributor to the impressive burst healing ability of the holy priest. The spell can be used directly following a [Prayer of Healing], healing 5 targets at virtually the same time (6 if glyphed).
The heal will not be a major hitter alone, healing for low amounts of healing on individual targets.
Being a "smart heal", the spell does very low amounts of overhealing. Many priests end up spamming the spell on cooldown due to it pushing priests high on healing meters as a result.
In TBC and in early WotLK, the dominant healing style of the Holy priest was to spam this spell mindlessly. The massive amount of healing done by "CoH-spam" trivialized a lot of encounters, which led Blizzard to add (and later increase) the spell's cooldown.
Patch 10.0.0 (2022-10-25): Now a row 4 Holy talent (was a level 39 Holy ability).
Patch 9.0.1 (2020-10-13): No longer a Talent and learned by all Holy Priests at level 39.
Patch 7.2.5 (2017-06-13): Circle of Healing healing increased by 15% and cost reduced by 40%.
Patch 7.0.3 (2016-07-19): Now does healing equal to 260% of spell power (up from 222%). Cooldown increased from 12 seconds to 15. Changed from level 50 ability to level 100 talent.
No longer prioritizes the most injured party or raid member.
Patch 5.0.4 (2012-08-28): Now a level 50 Holy specialization ability (was a tier 6 Holy talent).
Patch 4.0.6 (2011-02-08): Effectiveness has been increased by 30%.
Patch 4.0.1 (2010-10-12): Cooldown increased to 10 seconds. Moved to tier 6 (was tier 9).
Patch 3.1.0 (2009-04-14): Healing increased by approximately 40%.
Patch 3.0.8 (2009-01-20): Circle of Healing now has a 6 second cooldown.
Patch 3.0.2 (2008-10-14): Now works on any targets in the caster's raid, and is now a "smart" heal and chooses the lowest health targets to heal first within its range. Also now has a 6 second cooldown. Also no longer will heal summoned Snakes from Hunter's [Snake Trap].
Patch 2.3.0 (2007-11-13): Circle of Healing: The base amount of healing reduced, spell coefficient increased.
Patch 2.1.0 (2007-05-22): Circle of Healing effect increased.
Patch 2.0.10 (2007-03-06): Circle of Healing mana cost reduced by 25%.