semiformal

[sem-ee-fawr-muh l, sem-ahy-]

adjective

partly formal; containing some formal elements: a semiformal occasion; semiformal attire.

Origin of semiformal

First recorded in 1930–35; semi- + formal¹

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

The most enjoyable dinner is that with four or six guests, which is served in a simple and only semiformal way.
The Etiquette of To-day
Edith B. Ordway
The colored shirt is an innovation, and it should be used sparingly, white linen on any semiformal function being in better form.
The Complete Bachelor
Walter Germain
What additional formal or at least let us say semiformal instruction or education have you had in the Russian language?
Warren Commission (3 of 26): Hearings Vol. III (of 15)
The President's Commission on the Assassination of President Kennedy