Print spooler
A system service that accepts print jobs, buffers them on disk and feeds the printer at its own pace so the program need not wait.
A print spooler is the system service that sits between programs and the printer. Rather than letting a program drive the printer directly — slow, and one thing at a time — the spooler takes the whole job, parks it in a disk queue and sends it to the printer when it's free. The program is done at once; jobs print in turn.
The word spool is said to stand for "simultaneous peripheral operations on-line", a backronym that fit too well to resist. The idea dates to mainframes, where costly CPU time couldn't sit idle waiting on a line printer. Windows Print Spooler and Unix lpd/CUPS are the modern heirs — and a classic source of both stuck queues and security holes.