Transmembrane segment usually denotes a single transmembrane alpha helix of a transmembrane protein, also known as an integral protein. More broadly, a transmembrane segment is any three-dimensional protein structure which is thermodynamically stable in a membrane. This may be a single alpha helix, a transmembrane beta barrel, a beta-helix of gramicidin A, or any other structure. Transmembrane helices are usually about 20 amino acids in length.