Gardner’s definition:

Musical intelligence is the capacity to think in music, to be able to hear patterns, recognize them, remember them, and perhaps manipulate them.  People who have strong musical intelligence don’t just remember music easily – they can’t get it out of their minds, it’s so omnipresent.  Now, some people will say. “Yes, music is important, but it’s a talent, not an intelligence.”  And I say, “Fine, let’s call it a talent.   But, then we have to leave the word intelligent out of all discussions of human abilities.  You know, Mozart was damned smart!”