Usually a little box on a diagram that a solutions architect has drawn. This diagram will bare absolutely no resemblance to any kind of real software ever developed ever. Yet in the architects head - and in the minds of management they talk to it is a real and possible thing, with an arbitrary delivery date.
Developer: Oh no, Ray the architect wants me to build this bit of Architectware - apparently it will only take me a week.
Every other technically aware person on the team: Oh %*&£!