What is it called if fear of crowds?

Agoraphobia. Both. The greek translation is fear of the marketplace.
Agoraphobia. Agoraphobia is thought of as a fear of public places or crowds. One feels out of control in crowded places with ever increasing anxiety often leading to a panic attack. The logical consequence of this anxiety is to avoid such places and gradually one's world becomes smaller and smaller, even to the point of never leaving home. It can be a devastating illness, but there is treatment. Seek help.