Insubstantial

When a creature is insubstantial, it takes half damage from any damage source, including ongoing damage. See also half damage.

Published in Monster Manual 2, page(s) 218, Monster Manual, page(s) 282, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 312, Monster Vault: Threats to the Nentir Vale, page(s) 124.