Technically, a knot is a measure of distance, called a nautical mile, rather than speed. When you hear wind or speed as x knots, it actually refers to knots per hour.
The mile is known as a statute mile.
e.g., if you're flying at a speed of 100 knots, it's really 100 knots per hour, or about 115 miles per hour.
Technically speaking a knot is
a measure of speed (in addition to being something you tie in a piece of rope or string). In the olden days of sailing they would throw a weighted wooden paddle over the side of the ship. It was attached to a rope with knots tied in it at regular intervals. One person would hold the rope and count how many knots passed though his hand, in a set amount of time. That was how they determined their speed. You are correct that the modern knot is equal to a nautical mile per hour. While they both sound like they come from the same word, a knot refers to speed. In aviation you refer to distance in nautical miles (or just miles)