The noun fervor means a very strong feeling for or a belief in something.

Synonyms are ardor, passion, earnestness, or zeal.

The word origins from Old French (14th century) fervor meaning “heat, enthusiasm, ardor, passion” and Modern French (12th century) meaning ferveur. Another source is also from Latin fervor meaning “a boiling, violent heat, passion, ardor, fury”, and fervere meaning “to boil, be hot”.

The whole nation witnessed his patriotic fervor.

They moved from one geocache to another, searching for hidden treasures with fervor.

She was hypnotized by the fervor she found in his eyes.