Raglan sleeves
In a raglan sleeve the piece of fabric making up the sleeve doesn’t end at the shoulder, but goes all the way up to the collar or neckline. The sleeve is fitted to the rest of the garment with a diagonal seam that goes from the underarm straight to the neckline.
This means that they are wider in the underarm area and the sleeve fits more loosely than other sorts of sleeve. They also have no seam at the shoulder. Because of this Raglan sleeves allow a more natural range of movement and are very comfortable, making them ideal for sportswear.