Something that's cerulean is colored a clear, deep blue. On a sunny summer day, the sky is often cerulean, with just a few puffy white clouds.
Your cerulean sweater might be your favorite, since you believe it makes your eyes look cerulean as well. Tropical beaches can claim to have white sand and cerulean water, and a bright October morning often comes with a cerulean sky. The word has been around since the 17th century, from the Latin caeruleus, "dark blue," with the root caelum, "heaven" or "sky."