U

UHF - ultra high frequency
ultra violet - relatively harmful radiation with wavelength a little shorter than violet.
ultrasonic - sound with a frequency above the range of normal human hearing
umbra - full or complete shadow, as in an eclipse
Umbriel - moon of Uranus
uncertainty principle - principle that limits the detail (fineness) of measurement (Heisenberg)
uniform - steady, constant, as in 'uniform velocity' - constant velocity
unit - a 'description' following a number that distinguishes one quantity from another
unstable equilibrium - equilibrium where the object will not return to its original position if displaced
upper fixed point – the temperature at which pure water boils under normal atmospheric pressure
upthrust - upwards force on an object immersed in a fluid (e.g. water)
Ursa Major - circumpolar constellation, also known as the Great Bear or Plough