easter egg header
jargonA response header that carries a tribute, a joke or an invitation, placed where only people who read headers will find it.
HTTP lets a server send any header it likes, and a name beginning with X- was for decades the convention for a private one. Operators have used that freedom for things that are not protocol: Slashdot answered every request with a Futurama quote, Automattic hid a job offer, Airbnb's infrastructure team said hello, and after Terry Pratchett died in 2015 thousands of sites began sending his name in the overhead. None of it changes what the page does; all of it is found with one curl -I. The genre has three rules its best examples keep: it costs the reader nothing, it is true (a real invitation, a real tribute), and it is addressed to the people who would look.
Also known as: tribute header, hidden header, joke header, X- header easter egg