A knot is a securely fastened loop of string, rope, or fabric. Before kids learn to tie their shoelaces, they first have to learn to tie a simple knot. If someone asks you to "tie the knot," give it some thought since they are asking you to get married.
Sailors need to know how to make many different rope knots, and doctors learn to tie knots with the thin sutures they use to stitch wounds closed. You can also talk about a group or tight gathering of people as a knot: "A knot of fans formed around the movie star as she walked down the street." When you knot something, you tie it into knots or make a situation more complicated and snarled.