Y, 25th letter of the English alphabet. Like u, v, and w, it derives from the ancient Semitic letter waw (meaning “hook”), itself adapted from an Egyptian hieroglyphic. It passed into the Greek alphabet as the letter upsilon and was later used in the Roman alphabet only for words of Greek origin. In Old and Middle English, y was often used instead of i, a usage that is part of our language today. In chemistry, y is used as the symbol for the element yttrium.