Feathering is the migration of lipstick or gloss past the edge of the lips into the fine lines around the mouth, which turns a clean outline into a fuzzy, bleeding edge within an hour or two of application.
Also known as: lipstick bleeding, lip colour bleeding, lipstick feathering, gloss feathering
Feathering happens because lip colour is mostly oil and wax. Gloss is built on polybutene and emollient esters that stay liquid on the lip (Wikipedia), and warmth, talking and eating move that liquid outward. Vertical lines around the mouth act as tiny channels, so the colour wicks along them and the edge blurs. Dry, flaky lips make it worse, because the colour catches on flakes at the border.
A lip liner is the standard fix. A waxy, waterproof pencil drawn along the vermilion border forms a barrier that the oils do not cross, and it gives the colour an edge to stop at. Rare Beauty reports that 98 percent of testers agreed its Kind Words liner helps lipstick stay in place (Rare Beauty). Filling the whole lip with liner before colour adds a second dry layer for the gloss to grip.
Preparation reduces the amount of oil available to travel. Exfoliate, apply balm, then blot the balm before lining, so the pencil is drawn on a dry surface. Blot the colour once with a tissue before adding gloss, and keep gloss concentrated in the centre of the lip, away from the edge.
Formula choice matters too. Thin, high-oil glosses and lip oils feather fastest. A matte liquid lipstick, a stain or a liner worn alone feathers least, because there is little liquid left in the film once it sets. The fix for a feathered edge during the day is a cotton bud with a little concealer along the outside of the line, then a fresh stroke of liner.