Definition of "undermakeup"
undermakeup
noun
countable and uncountable, plural undermakeups
A layer of makeup that goes beneath the foundation.
Quotations
The undermakeup (primer) should be allowed to dry-set for 30 seconds. Apply foundation over the primer with a sponge using light, careful strokes to blend. Undermakeups come in cream form, sponge-on wands, or sponge-on cream, and as a lotion.
1989, Stan Place, Bobbi Ray Madry, The Art and Science of Professional Makeup, page 95
As the name suggests, an undermakeup toner is intended to be applied as a thin film to your face under your ordinary makeup. If, for example, you have a very ruddy complexion or have a dense network of tiny “broken” blood vessels on your face and nose (the so-called "drinker's nose"), you can use a green shaded undermakeup toner to reduce the flushed appearance.
2001, Nelson Novick, Saving Face