A style of shirt; a Henley shirt is a pull-over shirt without a collar but with a placket. The placket is most often styled with three or four buttons at the front of the neck, although some shirts have longer. It is essentially a golf shirt or polo without a collar. They are most commonly short-sleeved, but long-sleeved variants are available.
The Henley shirt is named after the shirts traditionally worn as part of the uniform of the well-known Henley-on-Thames rowing team. The style was popular among the general public for a while in the 1970s, and has maintained a presence in the fashion world ever since. The same general style was also used in undershirts, and this has remained common.
Currently these shirts are very popular as casual wear and sleepwear. In this case, they are usually heavier and warmer than a t-shirt, but artfully styled to show that it is not formal, with contrast topstitching (reinforcing stitches done in a contrasting color), and made of 'informal' fabrics such as fleece or waffle knit (AKA 'thermal knit').