A callus is a spot where your skin becomes rough and thick. After wearing flip flops every day, all summer long, you'll probably have a callus between your toes.
If you get a blister from playing tennis or digging in the garden, it will eventually turn into a callus if you keep swinging your racquet or using your trowel. Any spot on your skin that's rubbed and irritated repeatedly becomes a callus, or a thickened patch of skin. In medicine, another kind of callus is the bony tissue that forms when a broken bone heals. Don't confuse callus with callous — which sounds the same but means "insensitive and cruel."