Wiktionary
rhyme scheme
n. The pattern created by the rhymes at the ends of the lines of a stanza of poetry
Wikipedia
Rhyme scheme
A rhyme scheme is the pattern of rhymes at the end of each line of a poem or song. It is usually referred to by using letters to indicate which lines rhyme; lines designated with the same letter all rhyme with each other.
Bid me to weep, and I will weep While I have eyes to see And having none, yet I will keep A heart to weep for thee a b a b