A raglan sleeve is a sleeve which extends in one piece all the way up to the collar, rather than stopping at the shoulder as is the case with a conventional sleeve.
The raglan sleeve is fitted with a diagonal seam running from the armpit to the collarbone. The absence of a seam at the shoulder means that raglan sleeves allow more flexible movement and greater comfort, which makes them ideal for playing sport.
The name derives from the first Baron Raglan who lost an arm at the Battle of Waterloo. As a result of his injury, his coats were specially designed and incorporated this type of sleeve.