Snowball items

Unlike equipment experience, this involves simply incrementing a basic "use" counter and the item grows more powerful with each. Often a kill-anything-while-using-this-item counter, so you can fairly easily gain stacks on them. Likely said in a fashion of: +0.5 power per stack, up to 50 stacks, gain 1 stack per kill.

Some varieties of snowballing items lose the stacks when their user dies or is defeated otherwise, but these usually have smaller number of stacks than the others.

This does not cover items that temporarily gain stacks (e.g. gain stack per successful attack and lose them in 5-10 seconds after no successful attacks have been made).

In some games there are characters that have snowball traits (gain stats permanently for each player kill, especially when this gain has no upper cap; Dota2 and HoN have these, for example), but these are not part of this tag.