What is an antonym for the word billow?

Here are some antonyms for "billow," depending on the context:

If "billow" refers to a wave:

* Calm

* Still

* Flat

* Smooth

If "billow" refers to a large amount of something, like smoke or fabric:

* Shrink

* Contract

* Collapse

* Flatten

If "billow" is used figuratively to describe something growing or expanding:

* Diminish

* Decrease

* Contract

* Wane

The best antonym will depend on the specific context in which "billow" is being used.