Howdy Fly 33,
I have heard this term thrown around as well but you made me check my books. The other better informed than me may well correct me but I agree with you in that it is a speed.
The way I understand is that it is the speed where acceleration beyond that speed will result in a marked increase in drag. It also relates to the formation of a shockwave where the airflow decelerates below the speed of sound on the area aft of the max camber.
Below Mcrit (from my undesrstanding) there is no shock wave along the length of the airfoil. I have also read that the Citation X has a permanent shockwave over the wing that can be seen by the passengers and it looks like a lenticular cloud.
I'm not exactly sure of it's significance in this forum but would love to hear from anyone who wants to enlighten me.
Hope I've helped and not caused more confusion.
Ref