A collection of elementary particles which have approximately the same mass and the same quantum numbers except for charge, but have a sequence of charge values, (Y /2) - I, (Y /2) - I + 1, ..., (Y /2) + I times the proton charge, where Y is an integer known as the hypercharge, and I is an integer or half-integer known as the isospin; examples are the pions (Y = 0, I = 1) and the nucleons (Y = 1, I = 1/2). Also known as charge multiplet; particle multiplet.