From Lexicanum

Psychic Scream is a Tyranid Psychic Power.

In 3rd and 4th Edition Psychic Scream was a blast of psychic energy which distracted and scared any nearby units. Often this meant that it was less likely that the unit would do more heroic actions, such as staying in combat when overwhelmed by enemies.^1-p31

Sources

• ¹:Codex: Tyranids (4th Edition) (and Codex: Tyranids (3rd Edition) p9)