theory is on the Academic Word List. Pronunciation (UK) IPA (key): /ˈθɪəri/ or /ˈθiːəri/ or /ˈθɪri/ (US) IPA (key): /ˈθiːəri/ or /ˈθɪri/ Audio (US)(file) Noun Singular theory Plural theories Theory is on the Academic Vocabulary List. (countable) A theory is an idea that tries to explain an observation. The new information showed that his theory about growth was incorrect. Nobody has disproved Einstein's theory of general relativity. Related words theoristWikiwand - on Seamless Wikipedia browsing. On steroids.