To billow means to swell outward and fill with air, creating soft, rounded shapes that move and change. When you see sheets hanging on a clothesline on a windy day, they billow as the breeze catches them, puffing out like balloons before flattening again. Curtains billow through an open window. A ship's sails billow when they catch the wind.

Smoke and clouds also billow, rising and spreading in large, rolling masses. When a fire starts, smoke might billow from the windows. Storm clouds billow across the sky before a thunderstorm. The word captures that sense of expansion and movement, like something breathing in and out.

As a noun, a billow is one of those swelling shapes itself: a billow of smoke, a billow of fabric. The word also appears in descriptions of the ocean, where waves create billows as they roll and surge. There's something graceful about billowing, the way fabric or smoke or water moves in soft, flowing curves rather than sharp, sudden motions. When something billows, it looks almost alive, responding to invisible forces like wind or heat.