the shape of something rotating rapidly
"The tea leaves formed a tight swirl at the bottom of the cup as I stirred them quickly."
A whirling eddy.
"A small whirlpool formed near the boat's propeller, creating a swirling eddy that pulled at the waterlogged logs."
To twist or whirl, as an eddy.
"The tea leaves began to swirl in a small eddy near the center of the cup after I stopped stirring."