Jersey is the soft, stretchy fabric that is most often used to make T-shirts. Because they're usually made from this fabric, the tunic shirts that athletes wear are also called jerseys.
Jersey is made from knitted cotton, wool, or synthetic material, and it's used for a lot of commercial clothing — everything from polo shirts to fancy dresses. The name comes from Jersey, a region of what's now the British Isles where jersey was first developed. The word was first used for athletic shirts in the 1840s.