Peptide
Peptides (from the Greek πεπτός, "digested" from πέσσειν "to digest") are short polymers of amino acid monomers linked by peptide bonds, the covalent chemical bonds formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule. Peptides are distinguished from proteins on the basis of size, typically containing fewer than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond. There are also tripeptides, tetrapeptides, etc.
Amino acids which have been incorporated into a peptide are termed "residues"; every peptide has a N-terminus and C-terminus residue on the ends of the peptide (except for cyclic peptides). A polypeptide is a long, continuous, and unbranched peptide. Proteins consist of one or more polypeptides arranged in a biologically functional way and are often bound to cofactors, or other proteins.
The size boundaries that distinguish peptides, polypeptides, and proteins are arbitrary. Long peptides such as amyloid beta can be considered proteins, whereas small proteins such as insulin can be considered peptides.
Read more about Peptide: Peptide Classes, Peptides in Molecular Biology, Well-known Peptide Families in Humans, Notes On Terminology