A one-piece_swimsuit that has a sailor collar attached at the top portion.
Don't use this tag if there is a serafuku shirt over the swimsuit. For instance in post #482820, the middle character has a proper sailor one-pice, while the other two don't. Use the three tags serafuku, swimsuit, and swimsuit under clothes instead.
For the Idolmaster costume, see sailor swimsuit (idolmaster).
This tag implicates sailor_collar and one-piece_swimsuit (learn more).