"white line of ice crystals behind an airplane in motion," 1945, from condensation trail (1942); see condensation.
1777, "a projecting part of a rotating machinery used to impart motion to another part," from Dutch cam "cog of a wheel," originally "comb," from Proto-Germanic *kambaz "comb," from PIE root *gembh- "tooth, nail." It is thus a cognate of English comb (n.). This might have combined with English camber "having a slight arch;" or the whole thing could be from camber. It converts regular rotary motion into irregular, fast-and-slow rotary or reciprocal motion. "The original method was by cogs or teeth fixed or cut at certain points in the circumference or disc of a wheel ..." [OED]. Cam-shaft attested from 1850.
In the Ptolemaic system of astronomy, the tenth or outermost of the revolving spheres of the universe, which was supposed to revolve from east to west in twenty-four hours, and to carry the others along with it in its motion; hence, any great or first source of motion. [Century Dictionary]
A translation of Arabic al-muharrik al-awwal "the first moving" (Avicenna). Englished by Chaucer as the firste Moeuyng (c. 1400). Old science also had primum frigidum "pure cold: an elementary substance, according to the doctrine of Parmenides."