Other forms: armholes
An armhole is the opening in a shirt where your arm goes. It's not hard to get tangled in a dress if you get the armholes mixed up with the neck opening.
Sweaters, bathing suits, wedding gowns: these all have armholes. You might call these sleeves in the case of clothing that has them, although sleeveless garments — like chemises and sleeveless dresses — only have armholes. The original meaning of armhole, from the early 14th century, was "armpit" or "underarm," but the contemporary definition has come to be "opening in a garment for a wearer's arm."