Any obsolete or obscure device, equipment, piece of technology, or object, usually no longer being produced and seldom needed, with the exception of a single infrequent critical task.
Due to it's depreciated status, it is usually a near impossible item to acquire, however it is not necessarily expensive.
IT Guy 1: To get Harvey's favourite astrology program to work, I'll have to somehow get a hold of a 5.25'' floppy drive.
IT Guy 2: Oh man, where you gonna get a dodo egg like that? No one's making floppy drives anymore! Can't you just get him a newer program?
IT Guy 1: I wish, but he insists that no other program will do. Besides, he pays hourly.