See also semi- The Simple English Wikipedia has an article on: Semi-trailer truck Pronunciation (UK) IPA (key): /ˈsɛmi/ (US) IPA (key): /ˈsɛmi/ or /ˈsemaɪ/ Audio (AU)(file) Audio (UK)(file) Noun Singular semi Plural semis (countable) A semi is a kind of trailer that has no front wheels. Also, a truck pulling this kind of trailer. (countable) (slang) A semi is a semi-final. (countable) (slang) A semi is a semi-detached house. (vulgar) (slang) A semi is a partial erection. Wikiwand - on Seamless Wikipedia browsing. On steroids.