Full Definition of CONTEMPORANEOUS. : existing, occurring, or originating during the same time. — con·tem·po·ra·ne·ous·ly adverb. — con·tem·po·ra·ne·ous·ness noun. See contemporaneous defined for English-language learners.ull Definition of CONTEMPORANEOUS. : existing, occurring, or originating during the same time. — con·tem·po·ra·ne·ous·ly adverb. — con·tem·po·ra·ne·ous·ness noun. See contemporaneous defined for English-language learners.