The joint is a monaxial articulation at the elbow.

The humero-ulnar joint is a monaxial articulation at the elbow.

Humero-ulanr is a part of elbow's joint which allow the movement of flexion and extension in your joint.
Since the movement of both flexion and extension can only happens one way, the joint is categorized as a monaxial.

Edit your question hun so we know the choices