tropicyoudaoicibaDictYouDict[tropic 词源字典]
tropic: [14] The etymological notion underlying the word tropic is of ‘turning’, and the reason for its application to the hot regions of the world is that the two lines of latitude which bound them (the tropic of Cancer and the tropic of Capricorn) mark the points at which the sun reaches its zenith at the solstices and then ‘turns’ back. The word comes via Latin tropicus from Greek tropikós, a derivative of tropé ‘turning’ (source also of English trophy and related to the second syllable of contrive).
=> contrive, trophy, troubadour[tropic etymology, tropic origin, 英语词源]
tropic (n.)youdaoicibaDictYouDict
late 14c., "either of the two circles in the celestial sphere which describe the northernmost and southernmost points of the ecliptic," from Late Latin tropicus "of or pertaining to the solstice" (as a noun, "one of the tropics"), from Latin tropicus "pertaining to a turn," from Greek tropikos "of or pertaining to a turn or change; of or pertaining to the solstice" (as a noun, "the solstice," short for tropikos kyklos), from trope "a turning" (see trope).

The notion is of the point at which the sun "turns back" after reaching its northernmost or southernmost point in the sky. Extended 1520s to the corresponding latitudes on the earth's surface (23 degrees 28 minutes north and south); meaning "region between the tropics of Cancer and Capricorn" is from 1837.