Biological

From The StarCraft 2 Wiki

Biological units are those consisting mostly of "flesh and bone." All Zerg units are biological, as are Protoss and Terran infantry (Zealots and Marines). Biological units can be healed, but naturally cannot be repaired like mechanical units. Notable exception is the Terran SCV, which is considered both biological and mechanical and thus can be a target of any effect targeting either classification. Biological units are the "opposite" of robotic units.