A ball and a sport.
The ball is slightly smaller than a baseball. It is hollow, hard and white, and too light to be thrown or hit very hard or very fast. It is punched with holes. When I encounter wiffle balls, they usually have oblong holes in a radial pattern extending from one pole. However, I have seen one or two that have smaller circular holes punched in a regular pattern over the whole (ha ha!) surface. Wiffle balls sting at close range.
OUCH, that smarts!!
The sport usually ends up being much like baseball or softball, except with fewer gloves, fewer regulation sized fields, and fewer well defined rules...and practically no chewin' tobacco. However, there are more curbs, actual dinner plates, and at least as many heated arguments as in baseball. And you use a wiffle bat. Wiffle ball equipment is much more affordable than baseball equipment.