The first ingredient in a type of nail polish is "mer". What is this? Is it some kind of chemical? How is it used? How does it work?
In the organic lab, sometimes we would run reactions that produced a tar-like substance in the reaction flask as an undesired by-product. It was just a gooey polymer, and it happened often enough that this substance was just referred to as "mer", short for polymer. "Mer" was therefore a fairly common term used in many (most?) labs.
Since "mer" is listed as the first ingredient in nail polish, that indicates that it is the major component in the formulation. If you google "nail polish", you'll find a Wikipedia article. Its opening paragraph states:
Nail polish consists of an organic polymer with various additives
I suspect they use the term "mer" to refer to whatever polymer is the main proprietary ingredient in their formulation. The Wikipedia article also notes that
Nail polish consists of a film-forming polymer dissolved in a volatile organic solvent. Nitrocellulose that is dissolved in butyl acetate or ethyl acetate is common.
Once the volatile solvent evaporates, you are left with a hard polymer coating on your fingernail.