If you were to draw a Venn diagram, you'd see that polypeptides are a certain class of polyamides. Polypeptides are natural polyamides. Nylon is a synthetic polyamide. A polyamide is just a polymer formed from monomer units which lead to repeating amide bonds. Amino acids form repeating amide bonds to make polypeptides and proteins, which are natural oligoamides / polyamides.