Fly
Version 0.14: This article may not be up to date for the latest stable release of Crawl.
Fly is a status effect that sends you soaring above the ground. A flying player can ignore water, lava, and most traps, gains a bonus to stealth, receives twice the normal benefit of the Swiftness spell, and gets a small increase to speed if the character is a tengu.
Its only downside is that you take extra damage from Airstrike.
History
In 0.12 and earlier, flight reduced self-damage taken from static discharge, and negated additional damage from electrocution weapons.