Does “turn” refer to just the initiative a person acts on or the whole round?

@JeremyECrawford does "turn" refer to just the initiative a person acts on or the whole round? Mainly referring to sentinel feat.

— pickled pisces (@pickled_pisces) April 3, 2016

A round is composed of turns. They aren't the same thing. See "The Order of Combat" (PH, 189). #DnD https://t.co/PwMjlTxxnS

— Jeremy Crawford (@JeremyECrawford) April 4, 2016

@savevsBack in my day turns were composed of rounds. And we had Thac0! feh. I remember those days well!

— Jeremy Crawford (@JeremyECrawford) April 4, 2016