Knot has two main meanings:

1. A fastening made by tying rope, string, or cord together. When you tie your shoelaces, you're making a knot. Sailors use dozens of different knots, each designed for a specific purpose: the bowline creates a loop that won't slip, the square knot joins two ropes together, and the clove hitch secures a rope to a post. Some knots hold tight under pressure but release easily when needed. Others are meant to jam tight and never come loose. Learning to tie good knots matters for camping, sailing, climbing, and countless other activities. The phrase tie the knot means to get married, since tying knots symbolizes binding two things together.
2. A unit for measuring speed on water or in the air. One knot equals one nautical mile per hour. Ships and aircraft measure their speed in knots rather than miles per hour. A cruise ship might travel at 20 knots, while a fighter jet might fly at hundreds of knots.