An isopeptide bond is a type of amide bond formed between a carboxyl group of one amino acid and an amino group of another. An isopeptide bond is the linkage between the side chain amino or carboxyl group of one amino acid to the ¦Á-carboxyl, ¦Á-amino group, or the side chain of another amino acid. In a typical peptide bond, also known as eupeptide bond, the amide bond always forms between the ¦Á-carboxyl group of one amino acid and the ¦Á-amino group of the second amino acid. Isopeptide bonds are rarer than regular peptide bonds. Isopeptide bonds lead to branching in the primary sequence of a protein. Proteins formed from normal peptide bonds typically have a linear primary sequence.

Reference
Wiki: Formation of an isopeptide bond