... is one of the remedies that Dr Bach grouped together under the heading of 'Fear'. The Cherry Plum fear is very specific: it is the fear that one is going to lose control of oneself and do something dreadful, which can include injuring others and suicide.
Cherry Plum is also the remedy for the loss of control when it has in fact already taken place, and because because of the frantic fear and dread associated with loss of control. (Think of the fear of a small child in a screaming, irrational rage.) Fears of going mad and of acting irrationally are Cherry Plum fears.
Cherry Plum is one of the ingredients of the Rescue Remedy.