A sudden clever idea or inspiration, or an electrical wave produced by brain activity that can be measured.
Modern compound: 'brain' (Old English 'brægen') plus 'wave' (Old English 'wafian'). The metaphorical sense of 'a sudden idea' arose because brainwaves were discovered in the 1920s.
Your brain produces five types of waves (delta, theta, alpha, beta, gamma), and scientists can now tell if you're focused, relaxed, or sleeping just by measuring them—brainwaves are basically your brain's fingerprint!
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