From NexusWiki

ASV is an abbreviation used frequently by Nexus players, standing for Harden Armor, Sanctuary, Valor, three frequently used targetable buff spells. Harden Armor decreases AC by 10; Sanctuary lowers the amount of damage a player receives by 1/2; and Valor increases Might by 3. All three spells last for 180 seconds, or three minutes. Usually poets and mages only cast Harden Armor and Sanctuary, since the effects of Valor are unessential.

ASV is a significant staple of the game's combat, particularly in hunts. ASV is often recast or asked for as soon as the effect of the spell wears off, and the casters of ASV will often try to time their casts so that each player's ASV is cast (and, as a result, wears off) at around the same time, so that they do not have to worry about remembering to recast the spells for specific party members within the span of the next three minutes.

The three ASV spells are only available to the Mage and Poet paths; however, Rogues gain a non-targetable version of Valor called Might, which does not stack with Valor (and therefore, will not be casted on Rogues when ASV is cast).