u/APS_PS

Peptides are chains of amino acids connected by peptide bonds. Proteins are made from the same types of amino acids, but they are generally longer and more likely to form complex folded structures.
There is no single chain-length cut-off that applies in every case. Length, structure, and biological function can all influence whether a molecule is described as a peptide or a protein.
For people who study or teach biochemistry, which explanation is clearer for beginners: a rough chain-length cut-off or one based on structure and function?