Greater Healing Wave, usually abbreviated GHW, is a Shaman healing spell, which has a long casting time but restores a very large number of hit points. It is less mana efficient than [Healing Wave], but heals for much more with each cast, making it useful in situations where you need a lot of healing fast. Prior to Cataclysm, the spell was simply called "Healing Wave".
Together with Healing Wave, Greater Healing Wave is one of the two main single target healing spells in a shaman's arsenal. It is used in situations where Healing Wave alone is not enough to keep a target alive. Greater Healing Wave is too expensive to be chain-cast for longer durations, and should ideally only be used during periods of higher damage or to bring a target's health up quickly. When used together with [Tidal Waves], the spell can be used to quickly heal an injured target, but the long base casting time means it can be too slow to save someone when Tidal Waves is not active. In these cases, [Healing Surge] may be preferable.
At higher gear levels, the mana restoring potential of [Telluric Currents] means that Greater Healing Wave can be used as a shaman's main healing spell, although it still cannot be chain-cast for extended periods. Instead, Greater Healing Wave is used to keep a target stable, with Lightning Bolts used in between heals to keep mana high. Not using Healing Wave ensures a high Tidal Waves uptime, meaning that most or all Greater Healing Waves cast will benefit from it, freeing up more time for Lightning Bolts to regenerate mana. Because Telluric Currents is based on your spell power, this healing strategy is mainly used in endgame raid healing, especially on heroic difficulty.
Tips and tactics
Make a macro that activates [Ancestral Swiftness] and then casts Greater Healing Wave. That way you can can instantly land a very large heal with just one keystroke, saving valuable time in emergencies.
Tidal Waves makes Greater Healing Wave a great emergency spot heal, as it is much more mana efficient than Healing Surge. Try to avoid using Healing Surge for this purpose when you have Tidal Waves up.
Because restoration mastery benefits the entire heal based on the target's health when it lands, a single Greater Healing Wave will benefit more from mastery than several smaller heals in quick succession. This makes it especially effective for bringing people up from low health.
Healing and efficiency
The restoration specialization reduces the casting time by 0.5 seconds.