An extreme or irrational fear of something is known as a phobia, from the Greek word phobos 'fear'. There are various English words that describe the fear of particular objects, events, or situations. Some well-known examples include

claustrophobia - fear of confined spaces
agoraphobia - fear of open or public places
arachnophobia -fear of spiders
acrophobia - fear of heights
coulrophobia, fear of clowns
scopophobia, fear of being looked at
ailurophobia, fear of cats