A vector bundle over a Riemann surface is called stable (resp. semistable) if the ratio of its degree and its rank is bigger (resp. bigger or equal) than this ratio for any of its subbundles.

David Mumford, J. Fogarty, Frances Kirwan, (1994), Geometric invariant theory, Ergebnisse der Mathematik und ihrer Grenzgebiete (2) 34, 3rd ed. Springer 1994