A string of songs thrown together, often all by the same exact artist, to offer nothing more than a cheap masturbatory fantasy for fanboys and fangirls. A Megamix is often made by people who call themselves DJs but have no real skill or have never played in a club.
To further separate what makes a Megamix apart from a DJ Mix is that a Megamix will often forego all sense of flow, structure and common sense just to painfully string together the songs in any way possible.
* Vocal looping of the same line over and over,
* Using "jump edits" in the song to repeat a certain section of beat, note or part of bassline repeatedly, often causing it to sound like a CD skipping,
* Unnatural pitch increases to mix a 90bpm song together with a 140bpm song (instead of slowly scaling the tempo with other songs in between),
* Ignoring song cues and/or mismatching them,
* Overlapping two songs that aren't even in the same tempo or beatmatched,
* Not following the 4/4 structure or typical 16 or 32 beat dance mix structure,
* Cutting off one song in the middle of a verse or chorus just to make a hasty transition to another song that hasn't reached the same point in intensity of the previous track.
2) I went to a dance club and the DJ sounded like he was playing a Megamix, turns out he was really having a seizure in the booth.
(Only really applicable if you have a large collection including various wildly different genres; otherwise you've just got a shitty little vanilla playlist of a life.)
Micaela Superstar: "Idunno, just gonna put on the fuckin' Megamix."