Instant

From WoWWiki

The cast time of an instant cast spell is equivalent to a casting time of zero. In practice, instant means less than half a second but seems to depend on server response. Typically such spells can be cast on the move, which can serve as a great advantage over spells that must be cast while standing still. Instant cast spells come in many forms including DoTs such as Shadow Word: Pain and a Warlock's curses, heal-over-time spells such as Renew, and instant damage spells such as a Shaman's Earth Shock. Instant cast spells are distinct from channeling spells which cast instantly but can be interrupted before their full effect is complete.