Someone who feels attraction toward more than one gender.
Here's the twist most people miss: "bisexual" started life describing plants. From 1824 it meant an organism with both male and female parts — a hermaphrodite. It only got pointed at human desire in 1892, when the American neurologist Charles Gilbert Chaddock used it in his English translation of Krafft-Ebing's "Psychopathia Sexualis," a book that filed same-sex attraction under mental illness. So the label arrives pre-loaded with a clinic's fingerprints, not a community's.
The community took the word back. Through the 1960s onward, and loudly in the "Bisexual Manifesto" (widely dated 1990) out of the Bay Area Bisexual Network, bi people rewrote the definition on their own terms — refusing the "either/or" script that told them to pick a side. The most-cited modern definition comes from activist Robyn Ochs, who frames it as the capacity to be drawn to more than one gender — while stressing that this attraction need not arrive at once, land identically, or carry equal weight across those genders.
The persistent misconception is the "bi means two, so it's binary/transphobic" argument. Most bi organisations reject it: the "bi" contrasts attraction to your own gender AND to other genders — not "man or woman." Ochs' framing is explicitly non-binary-inclusive. A second myth worth killing: bisexuality is not a layover between gay and straight, and not a phase — it's a stable orientation that happens to get erased from both directions (the technical word is bi-erasure). A bi person married to someone of another gender is still bi.