optimismo

The -ism- machine, running on a bound root. -Ism- is a full official suffix in Esperanto and it does exactly what English -ism does: it names a doctrine, a system or a tendency. Attach it to a root and you get the belief: kapitalismo, socialismo, komunismo, nacionalismo, Esperantismo. Attach -ist- instead and you get the person who holds it, so optimisto is an optimist. Attach -a to that and you get optimisma, the adjective. All three are real, current Esperanto words and you can generate them without a dictionary.

What you cannot do is take the root away and be left with anything.Optim- is not a word in Esperanto. It is the Latin superlative optimus carried in whole because the international word arrived that way, and it appears only inside its own family: optimismo, optimisto, optimisma, optimume. That is normal for the -ism- words borrowed rather than built, and it does not weaken the suffix, which remains fully productive on native roots.

And its opposite comes free. Pesimismo, pesimisto, pesimisma, by the same three moves on the same pattern. The pair is worth learning together because the derivations line up exactly, which is the sort of tidiness that made people sit up in 1887.