* It represents a concept or idea: Idealism refers to a way of thinking or a belief system, not a concrete thing you can touch or see. It's a philosophical idea.
* It cannot be physically perceived: You can't hold or measure idealism. It exists in the realm of thoughts and beliefs.
* It usually ends in "-ism": Many abstract nouns, such as "realism," "capitalism," and "communism," end in "-ism." This suffix often indicates a system of ideas or beliefs.
Let me know if you'd like to explore other abstract nouns!