What is "mer", the first ingredient in a type nail polish?
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 &quot;mer&quot;, short for polymer.  &quot;Mer&quot; was therefore a fairly common term used in many (most?) labs.
Since &quot;mer&quot; is listed as the first ingredient in nail polish, that indicates that it is the major component in the formulation.  If you google &quot;nail polish&quot;, 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 &quot;mer&quot; 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.