Historically refers to a reel of multitrack tape upon which there is a submix of the tracks from a “master reel” to record overdubs against. The purpose of slave reels is to more easily provide additional workspace (tracks) for creating multitrack recordings. Back in the days when sessions began to span across two 24-track tape machines it was common to record the base tracks from the first machine as a mix on to the second machine. This allowed the overdubs to be recorded without having to sync up the two machines (which back then was a pain) for every pass of the tape. Once the overdubs were recorded using many tracks on the second machine, they were then mixed and bounced back to the first reel or were just mixed down to a comp track on the second reel. Nowadays our modern digital tape machines and disk systems have the ability to use what are known as virtual tracks (see WFTD archive Virtual Track) to accomplish almost the same thing.