knot: a join made by tying together two pieces or ends of string, rope, etc.
to tie a knot
Tie the two ropes together with a knot.
Tie a knot in the rope.
(figurative) hair full of knots and tangles (= twisted in a way that is difficult to comb)
a tangled knot of arms and legs
Sailors had to know lots of different knots.
Look at this balloon,
People blow air into the balloon & then tie its end to prevent the air escaping from it.
Do we call it a knot?