Singlet/ˈsɪŋɡlət/ Definition 1 of 4Pro Tip 1/2Casual Wear Singlets are informal and suitable for hot weather or sports activity. It's hot outside; maybe I'll wear a singlet to the beach. Pro Tip 2/2Layering Piece A singlet can be worn under other clothes for comfort or warmth. I always wear a cotton singlet beneath my sweater.