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The noun is derived from short (adjective) + wave (noun). The adjective is from an attributive use of the noun. The adverb is also derived from the noun.
Etymology from Wiktionary, CC BY-SA 4.0.
shortwave
short·waveToday it means: An electromagnetic wave with a relatively short wavelength; specifically, one with a wavelength generally between 10 and 100 metres, corresponding to a frequency between 3 and 30 megahertz. Full definition →