Capricious is an adjective to describe a person or thing that's impulsive and unpredictable, like a bride who suddenly leaves her groom standing at the wedding altar.

Is capriciousness a word?

adj. Characterized by, arising from, or subject to caprice; impulsive or unpredictable: capricious decisions; capricious weather. ca·pri′cious·ly adv. ca·pri′cious·ness n.

What is the definition of capriciousness?

: governed or characterized by caprice : impulsive, unpredictable.