The term "minicomputer" was first used in the marketing of PDP-8, but not immediately after its launch from 1965.

The use of "minicomputer" became widespread only in 1969, when it was explicitly derived from the word "miniskirt", which had become fashionable not long before that.

Quotation from "Electronic Design News", in July 1969:

“Design isn’t what it used to be. Today’s designer has available a ready-made solution to major problems — a minicomputer. Like the miniskirt, the small general-purpose computer presents the same basic commodity in a more appealing way.”

During the seventies, there was a classification of computers into mainframes like IBM System/370, supercomputers like Cray-1 and minicomputers like PDP-11.

Then in the second half of the seventies, "microcomputers" were added as the lowest tier.