To move gently and vertically, in either a single motion or repeatedly up and down, at or near the surface of a body of water, or similar medium.

“The ball, which we had thought lost, suddenly bobbed up out of the water.”

To move (something) as though it were bobbing in water.

“I bobbed my head under water and saw the goldfish.”

To curtsy.

To cut (hair) into a bob haircut.

“I got my hair bobbed. How do you like it?”

To shorten by cutting; to dock; to crop

To bobsleigh.

The motion of something that bobs.