Webster's Revised Unabridged Dictionary · 1913

Any irrational and persistent fear, whether of a situation, activity, or object; a phobia typically leads to a strong desire to avoid the object of the phobia.

A combining form used as a suffix to designate specific phobias, such as agoraphobia (fear of public places), acrophobia (fear of heights). The first part of the combining form is usually derived from Greek.