Line of Sight

From the Rulebook Glossary:
Verbal magic and abilities require that the target be visible to the caster upon completion of the incantation. If the target of the verbal magic or ability is wholly hidden outside of the casters view, such as [sic]short sword completely hidden behind a shield, then the magic or ability will be expended without affecting the target. If the caster can see a portion of the target, no matter how small, upon completion of the incantation, the spell will affect the target as normal. Thornwall and Forcewall block line of sight.

See Also: LOS Spells.