Someone attracted to people regardless of gender.
The word had a whole first life before it meant anything about who you fancy. Around 1914–1917, critics of Sigmund Freud coined "pan-sexualism" as an insult — their caricature of his idea that the sex drive powers basically all human behaviour. For years the word survived only as an insult aimed at Freudian psychology. The meaning we use today is a second life entirely, one that grew inside queer and BDSM communities and settled as a sexual identity in the 1990s. The desire came first; the label caught up.
The confusion everyone reaches for is "so what's the difference from bisexual?" The honest answer: they overlap heavily, and plenty of people use both for themselves. The usual distinction is emphasis. Bisexuality is attraction to more than one gender — gender is present, noticed, part of the picture. Pansexuality is more often framed as attraction regardless of gender — gender not weighed at all, including people who are trans, intersex, or outside the binary. GLAAD files pansexual under the "bi+ umbrella," which is a good tell: these are neighbours, not rivals.
How it actually shows up: it's an identity people claim for themselves, not a diagnosis you hand out. The pansexual pride flag — pink, yellow, blue stripes — was posted to Tumblr in 2010, a fair marker of when the identity went fully public. And the label describes a capacity, not a schedule: pansexual doesn't mean attracted to everyone, any more than straight means attracted to every person of another gender. It means the door isn't locked by gender.