“keen”

Characterized by intense enthusiasm or passionate devotion toward something.

“She was an ardent supporter of the expedition, volunteering before the call had even ended.”

Best for: Works well in formal writing or when describing deep, sustained enthusiasm rather than fleeting interest.

Showing a strong, eager desire or enthusiasm, often bordering on greediness for an experience.

“He was an avid reader, consuming three novels a week without pause.”

Best for: Slightly more vivid than 'keen'; fits journalism, profiles, and everyday elevated prose naturally.

Displaying intense, heartfelt enthusiasm or emotion that radiates outward.

“Her fervent interest in justice led her to law school by nineteen.”

Best for: Best used when the enthusiasm carries emotional heat; suits speeches, essays, and literary fiction.

Fiercely eager and energetically devoted, often to a cause or pursuit.

“The zealous young chef memorized every recipe in the kitchen within his first month.”

Best for: Carries a slightly intense or driven connotation; useful when enthusiasm borders on obsession.

Having a sharply perceptive, mentally acute quality — the intellectual sense of 'keen.'

“Her perspicacious eye caught the flaw in the argument that everyone else had missed.”

Best for: Use specifically for the 'sharp-minded' meaning of keen; ideal in academic or literary registers.

Intensely enthusiastic to the point of burning ardor, suggesting an almost feverish eagerness.

“His fervid curiosity about the cosmos kept him awake long past midnight, notebook in hand.”

Best for: Rarer than fervent and more dramatic; suits literary fiction and heightened rhetorical prose.