Why Do People Tell Actors To 'Break a Leg?'

The term 'break a leg' is unique to actors. It means, of course, 'Do well!' or 'Have a great show!' The term can be used before a stage performance, a show, or an audition. The expression reflects a theatrical superstition in which wishing a person 'good luck' is considered bad luck.
But where does the expression 'Break a leg' come from?