Primer (stress on "a") is the primary basis for make-up. The English word "primer" comes from the "prime" - primary. In its consistency, the primer is a clear liquid.
Primer smooths out the small roughness of the skin and allows you to get a more gendt surface, which creates the effect of young skin and fascinates men. Here, the same priming principle is used, that when painting on a wall or ceiling: first the surface is primed, and only then the paint is applied.
The primer is in the list: cosmetics