kilometer (n.)

word-forming element meaning "one thousand," introduced in French 1795, when the metric system was officially adopted there; irregularly reduced from Greek khilioi "thousand," from PIE *gheslo- "thousand," source also of Sanskrit sahasra-, Avestan hazanjra "thousand." "It is usually assumed that Lat. mille should be connected too" [Beekes]; see milli-. "In the metric system, kilo- means multiplied, & milli- divided, by 1000" [Fowler].

also metre, "fundamental unit of length of the metric system," originally intended to be one ten-millionth of the length of a quadrant of the meridian, 1797, from French mètre (18c.), from Greek metron "measure," from PIE root *me- (2) "to measure." Developed by French Academy of Sciences for system of weights and measures based on a decimal system originated 1670 by French clergyman Gabriel Mouton.

chiefly British English spelling of kilometer; also see -re.