The word 'billow' functions as both a verb and a noun.

Examples:

We watched the clouds billow in the breeze. (verb)

A billow of smoke from the campfire enveloped the campers. (noun)

The word 'billow' is both a noun (billow, billows) and a verb (billow, billows, billowing, billowed).

The noun billow is a singular, common, concrete noun; a word for a large undulating mass, such as a cloud, smoke, or steam; a rolling mass like a high wave; a word for a thing.

The verb to billow is to to fill with air and swell outward; to rise or roll in waves.