Agender is the lack of gender. It is a non-binary gender in which one is not male, female, or any other gender. While some agender individuals completely lack gender, some may call themselves agender because their gender is neutral or neutrois, because their gender is unknown or undefinable, because their gender does not align with any binary or non-binary categories, or because they do not care about gender as a label.
Agender vs Gender Neutral
There is little agreement about the difference between terms such as agender, gender neutral/neutrois. Some people use these terms interchangeably. For others there is a difference between these terms. It is often said that agender is the experience of having no gender at all, whereas gender neutral or neutrois is the experience of having a gender identity, a gender identity which is not male or female, but neutral.
Agender vs Gendervoid
Gendervoid, also called Voidgender, is similar to agender, and the two terms are sometimes used interchangeably. However, while they do overlap, they are not always the same. Agender is a more broad term that sometimes encompasses neutral genders. While an agender person may also feel as though they never had a gender in the first place, a gendervoid person may feel like there is an empty place where a gender would/should be but simply isn't.
The first recorded use of the word agender on the internet seems to come from a post on the site UseNet, on July 28, 2000. It was used in a discussion thread titled "alt.messianic". The user Miriam Wolfe wrote "All I understand is that G-d is amorphous, agender, etc. so "image" can't be a physical or gender or sexual thing." There were a few other instances of the word being used around the same time, but these also were either about God or were talking about people presenting in a gender-neutral manner (not as specific gender identity).
The first instance where the word was used about people as a specific identity seems to also come for UseNet, in a thread titled "alt.politics.democrats". The user man_in_black529 wrote "But despite this near-universality of gender, cultures can have transgender, agender, and hypergender individuals."
In both these cases the word agender was not defined, presumably, the definition was already known at the time. It's safe to assume that the word agender existed before these posts, and possibly had a similar meaning to what it does now. However, earlier uses of the word are not recorded.
The agender flag was designed by the Tumblr user Transrants in 2014. The color meanings are as follows: Black represents an absence of gender, and grey represents partial gender. Green represents the non-binary nature of this gender because green is the inverse of lavender (a mixture of pink and blue, meaning a mix of female and male).