The lower part of a bikini without the upper part (i.e., topless).
1965 March 12, “Topless Triumph”, in Time:
All this so impressed the Appellate Court in Aix-en-Provence that it reversed the convictions. "Inasmuch as the spectacle of the nudity of the human body has nothing intrinsic in it that would outrage normal, even delicate decency, and since Claudine Durand concealed her sexual parts with a sufficiently opaque monokini, we acquit her."
2009 July 22, Angelique Crisafis, “France Falls Out of Love with Topless Sunbathing”, in The Guardian:
French academics and historians have spent the early summer months pondering the sociological meaning of the demise of France's once-favourite piece of beachwear, the "monokini" – the bottom half of a bikini with no top.
2016, David Allyn, Make Love, Not War: The Sexual Revolution: An Unfettered History, →ISBN:
In fact, the San Francisco Chronicle featured a photo of a woman in a monokini — her exposed breasts clearly visible — on its front page.
A one-piece swimsuit, which may cover both the chest and the crotch (as contrasted with a bikini, which consists of two pieces).
2014, Joseph J. Darowski, The Ages of the X-Men: Essays on the Children of the Atom, →ISBN:
[Psylocke is] wearing a mix between a monokini and cut-out swimwear, a wardrobe change[.]