A knot is one nautical mile per hour and is the same as 1.852 km/hr (1.15 statute miles/hr), and a nautical mile is about a minute of arc (one-sixtieth of a degree) of the latitude or longitude of the Earth.
Consider the Earth as a perfect sphere and cut the circular cross-section into 360 equal sectors, gives a degree. Again dividing the degree into 60 equal sectors gives a minute of arc. So approximately, sixty nautical miles is a degree of circumference around the earth.
Then why named knot? Out there in the big vastness of the ocean, to measure how fast the ship is going was a difficult task in an earlier time. Thus around the 16th century, the navigators were using a technique called chip log or log-line to measure the speed of ships. A chip or a piece of wood (or any floating object) where one side weighs more than the other to keep the chip in a vertical position to increase the friction, later standardized into a quarter circle shape attached to one end of the rope where knots were tied at uniform intervals.
Sailors used to drop the log over the ship’s stern and let the log-line run out for a fixed time while counting the knots that passed over. The number of knots was treated as the speed of the vessel.
Featured Image: Blue cargo ship (PickPik)