To redden means to become red or to make something red. Your cheeks might redden when you're embarrassed, like when a teacher calls on you unexpectedly and you feel your face get hot and flushed. The sky reddens at sunset as the sun's light passes through more of the atmosphere. A ripe tomato reddens on the vine as it changes from green to its final bright color.

The word often describes involuntary changes. You can't always control when your face reddens from embarrassment, anger, or exertion. After running hard in PE class, students' faces typically redden from the effort. Someone might also redden from sunburn after spending too long outside without protection.

You can also redden something deliberately: an artist might redden a painting's background with crimson paint, or a cook might redden pasta sauce with extra tomato paste. The word captures both gradual change (like a sunset slowly reddening the clouds) and sudden shifts (like cheeks that redden instantly when someone hears surprising news).