A condensation reaction is a reaction in which two molecules combine to form one molecule, eliminating a small molecule, such as HCl or H₂O, in the process.

Peptide bond formation

Two amino acids can react with one another to form a dipeptide, linked by a peptide bond. A peptide bond is nothing more than a secondary amide functional group linking two amino acid residues. A water molecule is eliminated as a result.

In the illustrations below, two glycine molecules undergo condensation to form GlyGly (glycylglycine or 2-(2-aminoethanamido)ethanoic acid), expelling a water molecule.