A single strand followed through a larger body. Also a line of conversation held across interruptions.
Old English þrǣd (a twisted thread, a filament), from Proto-Germanic *thrēdus, from a root meaning to twist; a line of conversation held across interruptions is named for the single strand followed through a much larger cloth.
A single strand followed through a larger body, or a line of conversation held across interruptions. Both are the one twist that stays traceable through everything crossing it.