C2 [ C or U ] (a length of) a very thin fibre:

[ C ] a long, thin line of something such as light or smoke:

C2 [ C ] The thread of a book, discussion, speech, etc. is its story or the way that it develops, one part connecting with another:

C2 [ C ] a connected group of pieces of writing on the internet, where people are talking about a particular subject:

[ C ] a continuous raised line, such as the one that goes around the outside of a screw or bolt or the inside of a hole

C2 to put something long and thin such as string or thread through a narrow hole or into a small space:

to remove hair from someone's face using a long piece of thread: