Any song included on a compact disc single, besides the first song (the A-side).
Origin of b-side
From the B on the record's label
American Heritage Dictionary of the English Language, 5th Edition
B-side Sentence Examples
Internationally, it was released as a maxi single (similar to an EP, a maxi single with a single with more than one B-side).
For The Sake Of Greater Generality, The Days Of The Week Are Denoted By The First Seven Letters Of The Alphabet, A, B, C, D, E, F, G, Which Are Placed In The Calendar Beside The Days Of The Year, So That A Stands Opposite The First Day Of January, B Opposite The Second, And So On To G, Which Stands Opposite The Seventh; After Which A Returns To The Eighth, And So On Through The 365 Days Of The Year.