A well-known term, you might think, but the garment it referred to has changed considerably over time.
The ancestry of the corset is the renaissance bodice which, since around the mid 16th century, was stiffened with reed, later wood and other things. Around the end of the 17th it was found "boned", i.e. fitted with whalebone or, more accurately, baleen.
Throughout the late 17th and 18th centuries, the shape remained that of a funnel, i.e. tapering from chest to waist in a straight line, pushing up the breasts and releasing the hip by being cut into tassels from the waistline down. The straightness was often stressed by a busk being inserted into a pocket at the centre front.
While corsets nowadays have a rather bad reputation that was caused by the over-the-top waist fetishism of the late 19th century, 18th century corsets were not necessarily uncomfortable. They didn't so much compress the waist as re-shape the whole upper torso - actually you might find that in an 18th century corset, your waist circumference is a bit larger than the natural one, owing to 2 or more layers of fabric and the boning. The small waist effect is largely an optical trick, although there have always been victims of fashion who overdid lacing down.