The waterline is the moist rim of the eyelid between the lashes and the eyeball, where the openings of the meibomian oil glands lie, and the place where kohl and tightliner are applied to define the eye.
Also known as: water line, inner rim, lid margin, waterlijn
Each eyelid has a thin inner edge that touches the eye. That edge is the waterline: pink, damp, and lined with the openings of the meibomian glands that release the oil in tears. Make-up artists use it in two ways. Dark kohl on the lower waterline makes the eye look smaller and smokier. A nude, white or peach pencil on the same rim makes the eye look larger and brighter, the basis of the doe-eye look (Revlon). Pencil pushed into the upper waterline is tightlining.
Only soft, waxy formulas work here: a kohl pencil, a gel pencil or a waterline-specific tightliner. Liquid liner runs. Even the best claims are modest: MAC Colour Excess gel pencil promises 12 hours on waterlines against 24 or more on lids, because tears and blinking wear the product away.
Eye-care professionals are cautious. Optometrist Dr Alexa Hecht explains that product on the waterline blocks the meibomian glands; repeated blocking thickens the oil and contributes to dry eye, blepharitis and infection. Waterproof formulas and some preservatives are the most damaging, and her alternative is to draw the line under the lashes on the skin (Coveteur). Ophthalmologist Dr Rupa Wong puts it more bluntly: there is no waterline-safe eyeliner (Dr Rupa Wong).
If you line it anyway, sharpen the pencil before each use, keep it to yourself, replace it after 3 months, skip glitter and chunky pearl, and remove everything at night. Contact lens wearers apply liner after their lenses are in. Persistent redness, grit or crusting at the lash line is a reason to see an optician.