Bum-off-seat cycling
It can be called "standing sprint" if used by a road race bicyclist during a race/training. At MTB, it can be associated with "attack position", where one leaves the saddle momentarily (or for quite prolonged periods of time in fact, thanks @mattnz) to not be thrown out of the bike by the saddle launching the rider into space.

For regular commuters, Sheldon Brown considers the need to stand up while pedaling as a sign of a bad technique or poor bike fit:


  If you find yourself standing to accelerate, on level ground, it is a sign that your gear is too high or that your saddle is too low.


Also from there:


  Standing pedaling doesn't make you any faster, except in the very short run. 


So, check your gearing technique and bike fit.