Alphabetical. Also, of a poem, etc.: that has each verse or line beginning with a different letter of the alphabet, in alphabetical order.
Late 16th century; earliest use found in Abraham Fleming (c1552–1607), author, literary editor, and Church of England clergyman. From post-classical Latin abecedarius of or according to the alphabet, (as noun) learner of the alphabet from the names of the letters A B C D + classical Latin -ārius.