It's only a bit of trivia I suppose which I discovered several years ago and didn't get around to sharing it at the time. I do find this stuff interesting though so, hopefully, so do you.
In Susan Huxley’s book, “Sewing Secrets from the Fashion Industry” (pub. 1996 - great book, now out of print) she explains that “Whether a three-, four-, or five-thread version, the home-sewing industry usually calls them all sergers. Yet only a 5-thread serger, which produces an overlocked edge accompanied by a chain stitch seam, can accurately be called a serger.”
It's an American book describing something that didn't happen in the UK for some reason hence all our 'sergers' are either overlockers (with a prescribed number of thread capabilities) or combo machines (overlock/coverstitch).