A noun was a word which functioned as the name of a specific thing in a language.
In 2151, Hoshi Sato tried to translate things Klaang had said in Klingonese. She managed to translate most of it except for four words, which she speculated to be proper nouns: Jelik, Sarin, Rigel and Tholia. (ENT: "Broken Bow")
When a symbiotic lifeform had come aboard the Enterprise, Sato speculated she could communicate with it, but Captain Archer told her they weren't talking about simply nouns and verbs but a completely different form of language. (ENT: "Vox Sola")