Stomping is the act of stepping on a character with great force or weight, especially enough to cause bodily injury.
Usually a character will need to deliberately slam their foot down to get the force necessary to qualify as a stomp, but if a character is heavy enough, or there is significant size difference, simply walking on top of a character is more than enough force.
This tag makes sense to apply to objects and terrain too since it refers to a specific action a character does that has little to do with what is being stomped.