"Enthusiasm" is the noun form of "enthusiastic". Enthusiasm is a quality that people have sometimes. It means being positive, active, and ready to try something. You say that a person is "enthusiastic about" something. For example, when you're talking about a new project you're working on at work that you really like, you can say:
I'm feeling pretty enthusiastic about this one. What about you?
"Enthusiasm" is a noun that's made out of an adjective. Some other examples of this include:
- happy > happiness
- relieved > relief
- frustrated > frustration
- irritated > irritation
- certain > certainty
It's important to learn the noun forms of adjectives. Knowing them allows you to talk about different feelings and emotions that you or other people have.