A "waterline" in the context of women primarily refers to the inner rim of the eyelid where the eyelid meets the eyeball, also known as the area between the base of the lashes and the eyeball. It is a common term in makeup, where applying eyeliner to the waterline (called tightlining) enhances the eyes by making eyelashes look fuller or creating various eye makeup effects such as brightening or darkening the eyes

. In makeup application, the waterline is a sensitive area and requires careful use of appropriate products like waterproof or gel eyeliners to avoid irritation. The technique involves lining the inner rim of the upper or lower eyelid to achieve desired looks, such as smoky eyes or making eyes appear larger

. There is also a less common, slang usage where "waterline" can refer to the level of a dress or clothing that is so high it exposes intimate areas like genitals or buttocks, but this is not related to the makeup term

. In summary, for women, "waterline" most often means the inner eyelid rim used in eye makeup techniques, but it can also have a slang meaning related to clothing exposure. The makeup definition is the most widely recognized and used.