Omnigender is a gender similar to polygender, but more specific in what it means. Polygender is when one has multiple genders, while omnigender specifies that it is 'most, but not all' (those who have all being pangender).
History
Omnigender was coined by victomofrebellion on Tumblr.[1]
The pride flag was created by hyaenahart on Tumblr. The color meanings 'represent the binary and all genders inbetween.'[2]