I'm following the news about Libya and read in several places that coalition forces "flew 70 to 80 sorties on Monday, up from 60 on Sunday."

What is a sortie?

Is it the number of aircraft flown?
Number of bombs dropped from each aircraft?
What if it was a miss?
Count the Tomahawks, which I understand is launched from ships?
What if an aircraft encounters the enemy and has to engage?
Do the pilots keep track of his/her own sorties?

70 to 80 sorties implies that command doesn't know exactly how many?