brainwave vs theta wave

brainwave and theta wave both are nouns.

Word	Noun	Adjective	Verb	Adverb
brainwave	Yes	No	No	No
theta wave	Yes	No	No	No

As nouns, theta wave is a hyponym of brainwave; that is, theta wave is a word with a more specific, narrower meaning than brainwave:

brainwave: (neurophysiology) rapid fluctuations of voltage between parts of the cerebral cortex that are detectable with an electroencephalograph
theta wave: the normal brainwave in the encephalogram of a person who is awake but relaxed and drowsy; occurs with low frequency and low amplitude

Other hyponyms of brainwave include alpha rhythm, alpha wave, beta rhythm, beta wave, delta rhythm, delta wave, theta rhythm.

brainwave (noun)	theta wave (noun)
the clear (and often sudden) understanding of a complex situation	the normal brainwave in the encephalogram of a person who is awake but relaxed and drowsy; occurs with low frequency and low amplitude
(neurophysiology) rapid fluctuations of voltage between parts of the cerebral cortex that are detectable with an electroencephalograph