The continuous covalent path through a molecule. For polypeptides, the backbone includes all peptide bonds, alternating with alpha carbons, plus disulphide bonds. for polynucleotides, the backbone includes all phosphodiester bonds, alternating with the C3'-C4'carbons in the sugar (ribose or deoxyribose).
In protein, the continuous chain of amino nitrogens, alpha carbons and carbonyl carbons in the polypeptide chain (i.e. everything except the amino acid side chains).