English has four open word classes: nouns, verbs, adjectives and adverbs. The reliable way to tell them apart is a slot test rather than a meaning test — a noun fits “the ___ is here”, a verb fits “they ___ed yesterday”, an adjective fits “a very ___ thing”, an adverb fits “she did it ___”. Meaning-based definitions fail immediately: “arrival” is a noun and it is not a person, place or thing.
Four classes, and the ones that are missing
This dictionary records four parts of speech: 117,953 noun entries, 21,499 adjective entries, 11,540 verb entries and 4,475 adverb entries. Those add to 155,467 across only 147,478 entries, because a great many words belong to more than one class.
They are the four open classes — open because new members join constantly. English gained new nouns and verbs this year and will gain more next year. What is missing is the closed classes: determiners (the, a, this), prepositions (of, in, between), pronouns (she, which), conjunctions (and, because) and auxiliaries (have, will). They are missing because WordNet is a database of lexical meaning, and those words do grammatical work rather than carrying a meaning you could gloss.
That is a real limit and worth stating plainly. If you look up “the” here you will not find a determiner entry, and that is not an oversight in the corpus — it is the boundary of what the corpus is for.