pullover
A sweater or top you pull over your head.
A pullover is a piece of clothing you put on by pulling it over your head, rather than buttoning or zipping it up the front. Sweaters, sweatshirts, and some shirts are pullovers. The name tells you exactly how to wear it: you literally pull it over your head and arms.
When your grandmother knits you a sweater for your birthday, she's probably making a pullover. When you grab a hoodie on a chilly morning, that's likely a pullover too (unless it has a zipper down the front, which would make it a zip-up instead).
The opposite of a pullover is a cardigan, which is a sweater that opens in the front with buttons or a zipper. Some people prefer pullovers because they're simpler and keep you warmer with no opening in the front. Others like cardigans because they're easier to take on and off without messing up your hair.