Midgender is a gender that exists in between or in the middle of the binary genders (female and male). It can be considered similar to the original definition of intergender, while not being intersex exclusive. Similar terms include androgyne and lingender.
Midgender was coined by DeviantArt user NaturallyDarkV on or before June 19th, 2017. Mid- comes from the word midpoint.
The flags were coined by Tumblr user b3anie and Mod Hermy from beyond-mogai-pride-flags on June 19th, 2017. It shows a blending of blues and pinks to represent it being midway between female and male.