What is the part of speech billowing?

"Billowing" is most often used as a verb, meaning to swell out or expand like a sail in the wind.

However, it can also be used as an adjective, meaning "expanding or swelling in a rounded or voluminous way."

Here are examples of both uses:

Verb: The smoke billowed from the chimney.

Adjective: The billowing clouds filled the sky.