Can't say when or who came up with the term chiptune, but it definitely was around 89/90. I was a scener back in 90-92, and the term did mean Amiga music that used tiny (often looped) samples that sounded like synthesizer sounds. Like a synthesizer chip that the Amiga's sound chip was not. Of course the snare drums and so on were a bit longer samples, as there was no real synthesis to make them (and software synthesis must be pretty CPU intensive on an A500).
I would guess the first primary use for chiptunes was crack intros - they had to use minimum disk space so the games would still fit on the disks. Then they evolved into an art of its own - like the chiptune musicdisks from 4Mat and so on.
I would call "a protracker tune that has 14 samples and 1 lead instrument as a triangle wave" a chiptune, as that's the way a lot of the so called chiptunes were made. Of course there was the emphasis on making all the other samples as short as possible, too.
I guess the term is a bit loose, we all know chiptunes by the sound...