A sweater was a piece of clothing, meant to keep the wearer warm while outside or inside during cold temperatures.
In 2368, after Lieutenant Commander Albert gave Cadet Wesley Crusher his sweater back following his son, Joshua Albert's death, Crusher remarked that he had loaned it to Albert during their skiing trip to Calgary, Canada, since Albert forgot his. (TNG: "The First Duty")
After reuniting with his former Starfleet Academy girlfriend Lieutenant B'Elanna Torres on the USS Voyager, Lieutenant Maxwell Burke asked Torres where his Academy sweater that he had given her had gone. (VOY: "Equinox")