Fervency is an intense, passionate feeling. Your fervency for your favorite football team is clear from the way you jump around and yell as you watch their games on TV.
When you intensely adore someone, you love them with a kind of fervency. Other fierce emotions, especially warm, positive feelings, can also be called fervency, like your fervency for rescue dogs or the fervency of a preacher's Sunday sermon. The Latin root of this word means "to boil" or "to glow," so when you experience an emotion so passionate that you feel like you're glowing — that's fervency.