an MPEG picture that is Bi-directionally predicted from a combination of a preceding reference frame picture or field picture and a following reference frame picture or field picture.  Each reference is either an I picture or a P picture.  B pictures are never used for prediction of other pictures. Several B pictures may appear in sequence between I and/or P pictures. (See Frames, Fields, Pictures (I, P, B) , also MPEG 2 Prediction )