Technically speaking, this ‘bikini line’ is called the Hypogastrium (alternative names including hypogastric region, pubic region and suprapubic region). It is derived from the Greek word meaning ‘below the stomach’.
The hypogastrium is the line (here denoted in black) used to perform Caesarian sections.
In anatomical terms, the ‘bikini area’ could be defined as the subcutaneous part of hypogastric and inguinal (right and left) regions below the line connecting the anterior inferior iliac spines*.
The bikini line is less prominent in underweight women, and its presence was favored as a sign of affluence and sexual allure in Ancient Greece.