Omnisexuality is a sexual orientation where the individual recognizes and is attracted to people of all sexes, genders, and gender identities, with gender as a factor in their attraction. This includes people who identify as agender.
Some people who identify as omnisexual may have a preference to which gender they feel most attracted to. For others they may be equally attracted to all genders, but they may feel the act of being attracted to one gender feels different than the act of being attracted to another gender. They might also find certain traits only attractive in certain genders.
The prefix omni- comes from the Latin word omnis, meaning "all".