A bikini was an item of clothing.
Whilst travelling to Florana, Sarah Jane Smith wore a bikini. (TV: Death to the Daleks)
The Eleventh Doctor swapped places with with a stripper called Lucy who was to burst out of a cake for Rory Williams' stag night. After bursting out of the cake, he said that she was outside in a bikini and asked if someone could let her in and give her a jumper as she was a lovely girl and diabetic. (TV: The Vampires of Venice)