Knot

Most people recognize the everyday meaning of the noun 'a knot' (and its related verb 'to knot', 'to make a knot', 'to tie') - an arrangement of string or rope that serves to fasten it firmly to another piece, or a post etc; occasionally just to make the rope thicker. ("An intertwining or complication of the parts of one or more ropes, cords, or strips of anything flexible enough, made for the purpose of fastening them together or to another object, or to prevent slipping, and secured by being drawn tight", OED). But it may be of use to readers of AWE to point out several subtleties in other meanings.

• • • The phrase 'to tie the knot is a colloquial expression for 'to marry'.
    • To 'tie a knot in one's handkerchief' is to make a reminder to oneself.
    • A 'knot under one's ear' refers to the hangman's knot used to make the noose with which those condemned to death were executed.
    • A knotty problem is a very complicated and intricate one, whose solution or construction may be hard to trace.
• In the days of sailing ships, many arrangements of cords were used, often with specific purposes. Various sub-categories of knots were recognized:
  • bends were used to connect or link cords of the same or different sizes;
  • hitches were used to fasten ropes to other objects.
  • In this strictest sense, knots should be large decorative complications at the end (sometimes in the middle) of a rope, to stop it, or block it from running away, or slipping from a hand. The decorative effect was adopted in various applied arts, where knots can be
    • finials, in stone or wood carving;
    • conventional patterns in heraldry, and forms in manuscript decoration ('celtic' ornamentation);
    • flower beds, in gardening - knot gardens were popular in Tudor England.
• Several other meanings derive mostly from from this strict sense, in the general sense of 'thickening' or 'blocking':
  • Figuratively a 'lump', 'hardening or thickening'. With muscles, this indicates tensing, as a knot in jaw muscles may indicate an effort of self-control, or a knot in the stomach may be a sign of fear; a small group or cluster forming in a wider crowd can be called 'a knot'. In wood, a knot is the circular thickening, often of a dark colour, left in the trunk of a tree by the branch which has grown out of it.
  • The unit of measurement of speed customarily used on ships is the knot: one knot = one nautical mile per hour. (The nautical mile is a unit fixed in 1929 at exactly 1,852 metres (approx. 6,076 feet, 1.151 statute miles): in Britain before 1929, the nautical mile was fixed by the Admiralty at 6,080 feet .)

To talk of 'knots per hour' is an error to be avoided.  (It would be a unit of acceleration). 
      The knot is already a unit of speed.

Etymological note: the best measurement of speed through the water that could be achieved in the past was by 'casting the log'. This was a piece of wood attached to a free-running line. The 'log' (later not just a lump of wood, but a quadrant of wood, crafted to give maximum resistance to the water) was thrown overboard. This dragged line off the reel for a measured time (originally 28 seconds, measured by sand-glass). The line was marked at precise intervals with knots (later 'tags' or 'marks'), coded to allow readings to be taken even in the dark.

• In mathematics, a knot is "a closed curve which does not intersect with itself, formed by looping and interweaving the curve with itself, and joining the two ends together, so that it cannot be untangled to form a simple loop" (Clapham and Nicholson 2009)

• In ornithology, knot is also the name of a bird, the red-crested sandpiper (Calidris canutus; formerly Tringa canutus). (In North America, it is called 'The Red Knot'.) There is also a Great Knot, which breeds in Siberia and winters as far south as Australia.