Sweater and sweatshirt are the U.S. English terms for garments worn on the upper body. The difference between the two types of garments is primarily the materials that they are made from, and to understand these more fully, we need to look at their history.
According to Etymonline.com, the term ‘sweater‘ originated in the 16th century to describe ‘one who works hard’. By 1828, ‘sweater’ referred to clothing worn to produce sweating and reduce weight, and by the 1880s, described woollen vests worn by rowers.
In British English, woollen sweaters are known as ‘jumpers’, from the French ‘juppe’ or ‘jupon’, which in medieval times referred to a short coat (not to be confused with the modern French ‘jupe’ which means skirt).
Made from knitting or crocheting wool, sweaters have become desirable fashion items and come in a full range of styles, from short cardigans to long coat-like garments, and from shawls with sleeves to hooded pullover styles. Sweaters can be delicate and made from thin wool, or they can be thick and chunky. Womens Irish sweaters, for example, would refer to garments made from thick wool, with the knit incorporating thick, cord-like patterns. Examples of these can be found at specialist outlets such as https://shoparan.com/womens-irish-sweaters/.
Sweatshirts, on the other hand, can be made from any type of material, such as cotton or polyester. The term appeared in 1925 and referred to a long-sleeved pullover made of fleecy cotton, designed for athletes to wear during training.
Like sweaters, sweatshirts were developed primarily for warmth, but there the similarity stops. The fleecy lining and smooth outer shell are designed to absorb sweat and are used as sports outerwear and sports fashion wear.
As such, the range of styles in sweatshirts is much more subtle. Almost all feature elasticated cuffs and a band at the bottom, with the most notable differences between styles being the optional inclusion of a hood, or a half or full zip.
Decoration on sweatshirts tends to be embroidered or printed, which makes the garment a popular choice for company-branded outdoor workwear.