Other forms: T-shirts
A T-shirt is a casual, collarless shirt. Your date won't be happy if you show up at the formal dance wearing a T-shirt and jeans.
You can also call a T-shirt a tee shirt, or simply a tee. Most of your favorite casual, lightweight shirts are probably T-shirts, including the printed ones you buy on vacation and the ones that proclaim your favorite teams or musicians. The T-shirt evolved from underwear, and it got its name around 1920 from the T shape it forms when you lay it out flat.