I don't know if you CAN define a dyno - but you know when you're doing one. Also it depends whether you're vobaculary also has words such as "snatch", "lunge", "slap" etc. Some people don't and thus use dyno to describe any non-static move. I also notice a lot of guidebok decriptions use the word "Dyno" rather too frequently to try and spice-up routes descriptions, when they really mean its a lunge or slap.
I say if your feet are off, then its deffinetly a dyno. But thats not to say that if you've still got a foot on then its not a dyno, cos some you might.