A shortfill is a bottle of flavoured liquid for refilling ecigs. They do not contain nicotine, so can be sold in greater volume than 10ml. (see TPD)
Shortfills are so named because they are deliberately filled allowing room for the addition of nicotine as a Nic Shot, or several as required. Typically they are sized so that one nic shot gives 3mg/ml (50ml + 10ml of 18mg/ml liquid) gives a total volume of 60ml at 3mg/ml.
The reason it is called a shortfill is that the 60ml bottle will only be filled to 50ml to allow for room for the 10ml of nicotine liquid (in the example above).
There is a handy onlineNicotine shot calculator that takes all the hard math out of it.