A Maillot is a one piece bathing suit and was first used in the 1920s to describe tight fitting swimwear. Remember, bathing suits in the earlier part of the 20th century looked more like sundresses with full skirts and sleeveless styles.
Usually when I see the word "Maillot" to describe a bathing suit I imagine a strapless or heart shaped bust bathing suit or one with vintage inspired details such as ruching at the waist. To me, a Maillot is synonymous with the bathing suit styles of the 1950s.
Now a days we just call it a one piece. But if you want to be a bit more fancy, you can say you're planning on sun bathing in your gorgeous maillot bathing suit instead of sunbathing in your one piece. *hehe*