main line mainline • mainly · · · Info · Definitions · Characters · · main line · · Text Common · · Latin · · Scripts 9 · · Length 2 · · Syllables Definitions · · · (noun) the principal route of a transportation system. Synonyms: main line. · · Noun Definition 1 (verb) inject into the vein. Synonyms: mainline. Example: "She is mainlining heroin." · · Verb Definition 1 Characters · · space U+0020 a latin small letter a U+0061 e latin small letter e U+0065 i latin small letter i U+0069 l latin small letter l U+006C m latin small letter m U+006D n latin small letter n U+006E