Good question. The term is sometimes misused or misinterpreted, so it’s worth getting clear on what’s what.

When an album is recorded, whether in the studio or on stage, it will either be a multi-track recording using several microphones and/or different sessions to record each individual element separately (instruments, vocals), or a live or semi-live recording in which one or several microphones capture the performance as a whole. The recording engineers will then bring the various elements together, editing and mixing them to achieve the desired sound and to remove unwanted noise, culminating in the album’s first final arrangement. This is the original studio master tape, of which there will be not just one, but several: each of these is considered an ‘original master’.

This original master is then used as the ‘blueprint’ for all subsequent copies, pressings, remasterings, etc. But of course with each subsequent treatment, something of the original information and hence sound quality is lost. Which is why nothing sounds quite like the original studio master tape. It’s the original source of the album in its completed state.