substantivo

This is a metalanguage word: you need it to talk about Esperanto rather than to talk in it. Substantivo is a noun, the part of speech, and it joins verbo at rank 1,714 in the small terminology set the course keeps handing you.

That set is worth seeing as a set, because it is unusually regular. Substantivo, adjektivo, verbo, adverbo, prepozicio, konjunkcio, pronomo. All borrowed from the same Latin grammatical tradition, all with predictable Esperanto endings bolted on. Once you know three of them you can guess the rest, and be right.

The reason Esperanto grammar terminology is easier than most is that the language wears its parts of speech on its sleeve. Every word ending tells you the class: -o is a substantivo, -a an adjektivo, -i a verbo in the infinitive, -e an adverbo. In English you have to parse the sentence to know whether run is a noun or a verb. In Esperanto the word has already told you, and substantivo is the label for the largest of those classes rather than a category you have to work out.