Both zit and pimple refer to the same thing: a small, red, swollen spot (or “inflamed elevation,” if you’re fancy) on the skin. Zit is generally considered an informal or slangy synonym of pimple, which itself is less formal, medically speaking, than either papule or pustule.
Of the two words, pimple is the oldest by far, first used in the 14th century and sharing roots with the Old English verb piplian, meaning “to break out in pimples.” Zit is much more recent; its first-known use in print popped up (sorry) in a 1965 newspaper advertisement for acne treatment in the Sacramento Bee. Its origins are unknown.
A young man from a high school near here, who made a specialty of mathematics and pimples, and who could readily tell how long a shadow a nine-pound groundhog would cast at 2 o'clock and 37 minutes P.M., on groundhog day, if sunny, at the town of Fungus, Dak. …
— Edgar Wilson Nye, Bill Nye’s Sparks, 1891“Well, he does have glasses and braces and zits but other than that, he’s really cute.”
— D. J. Tanner (played by Candace Cameron), Full House, 1988
Acne, for what it’s worth, refers to a disorder of the skin caused by inflammation of the skin glands and hair follicles, and especially a form of this disorder found chiefly in teenagers and marked by pimples/zits on the face.
