If the amount or size of something swells or if something swells it, it becomes larger than it was before.

If something such as a part of your body swells, it becomes larger and rounder than normal.

If you swell with a feeling, you are suddenly full of that feeling.

If sounds swell, they get louder.

A swell is the regular movement of waves up and down in the open sea.

You can describe something as swell if you think it is really nice.