Overview
According to wikipedia, a bikini is defined as a woman swimsuit in which one part of the attire covers the breasts and the other part covers the groin and part of or the entire buttocks, leaving an uncovered area between the two.
It is also said that the modern bikini was invented by French engineer Louis Réard in 1946. He named it after Bikini Atoll in the Pacific, the site of the Operation Crossroads nuclear weapon tests in July that year.
Usage
In most anime, there is a common trope where showing a bikini in public (specially when there's a beach scene) is normal while exposing undergarments (bra + panties) is considered an embarrassing moment. However, some characters always feel uncomfortable wearing a bikini, even on beaches.
Disambiguation
Bikini is one of the characters from Ace Attorney series. Please refer to the link for more details.