"apparent displacement of an object observed, due to an actual displacement of the observer," 1570s, from French parallaxe (mid-16c.), from Greek parallaxis "change, alteration, inclination of two lines meeting at an angle," from parallassein "to alter, make things alternate," from para- (see para- (1)) + allassein "to change," from allos "other" (from PIE root *al- "beyond"). Related: Parallactic.
1889 as an abbreviation of British Thermal Unit (1862), a commercial unit of electrical energy (the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit); the French Thermal Unit is the amount of heat required to raise 1 kilogram of water 1 degree centigrade. Also from 1889 as an abbreviation of Board of Trade Unit, in electicity "1,000 watt hours."
1610s, "unity, arithmetical unit," 1610s, from Late Latin monas (genitive monadis), from Greek monas "unit," from monos "alone" (from PIE root *men- (4) "small, isolated"). In Leibnitz's philosophy, "an ultimate unit of being, a unit of the universal substance" (1748); he apparently adopted the word from Giordano Bruno's 16c. metaphysics, where it referred to a hypothetical primary indivisible substance at once material and spiritual. Related: Monadic; monadism.
