A spool is a bobbin: a cylinder or reel for winding yarn, thread, cable, film, etc.

In computing, a spool is a storage device that incorporates a physical spool, such as a tape drive. The term is also used metaphorically for a storage area that is accessed in a spool-like manner: that is, material is only added and deleted at the ends of the area; there is no random access or editing. For example, the temporary storage area to which email is delivered by a Mail Transfer Agent and in which it waits to be picked up by a Mail User Agent is sometimes called a mail spool. See also: queue.

Spool is also an acronym for 'Simultaneous Peripheral Operation OnLine'. It refers to the use of a memory buffer to store communication between the CPU and peripherals, in order to allow the CPU to perform another task while the (relatively) slow peripheral handles its task.