"Knots"
In th'old days a "bare chested seaman" [oo-eerr Matron] stood at the edge of the ship/boat/whatever. The Captain/navigatot started his "stopwatch" [eggtimer] and said "NOW". Whereupon our chappie let loose a "LOG" [a piece of wood] attatche to him by a bit of string....However along this bit of string there were tied KNOTS. At the end of the timed run, our hero woud say "Seven and a half knots, zurr" or somesuch. Hence Knots.. [K] Nautical Miles per Hour.
TAS, True AirSpeed
IAS, Indicated A/S
CAS, Corrected A/S
Mach, a percentage of the "speed of sound" Holds true no matter what Height/Ground/Air Speed you have.
eg M1 [Mach1] at "Sea Level" is [approx] 760 MPH
At 30,00ft is [approx] 630 MPH
Concordes' M2.2 [1440mph] was an "Airspeed" of [approx] 750MPH
I stand to be corrected of course [The USSR etc use KPH
and Metres for speed and Altitude]
watp,iktch