A seventy-eight is an old kind of music record: a hard disc that spins on the player about 78 times each minute. People also call them 78s. Before later, slower records came in, this was the usual record in the home. They are thick, they break if you drop them, and each side holds only a few minutes of sound. You will not buy new pop music this way now; you meet the word in antique shops, in family cupboards, and when someone talks about early jazz or very old recordings. Though actually, seventy-eight is also just the number 78 — but this meaning is the disc, named only for that speed (78 rpm). Later records ran at 45 or 33 times a minute; those are different objects with different names.