The word tropical was first used in English in the 1520s when someone added the suffix -al to the pre-existing word tropic. At the time, it only referred to the geographic zones, but by the late seventeenth century it came to mean any place that was humid like the tropics. Tropic in its purest form refers to the most north or south latitude where the sun can still be directly overhead, and obviously this was associated with warmer climates over time to have a more general meaning. The word comes from Latin tropicus, "pertaining to the solstice" (this zenith can only occur on the solstices for the tropics of Cancer and Capricorn), and that's from Ancient Greek tropikos, which had to do with changes in general. That derives from trope, meaning "turn" (and etymon of English trope), which is reconstructed to PIE trep, also "turn".
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