Biological

From The StarCraft 2 Wiki

All Zerg are biological units. That means units that are close to 100% made out of flesh and bone. Protoss and Terran infantry are also biological (Zealots and Marines). Biological units can be healed, but naturally never repaired like mechanical units. They are the complete opposite of robotic units.

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. They are the "opposite" of robotic units.