“Organizational citizenship behavior (OCB), defined… as behavior that (a) goes beyond the basic requirements of the job, (b) is to a large extent discretionary, and (c) is of benefit to the organization” (Lambert, S.J., 2006, p. 503-525).
“OCBs are employee behaviors that, although not critical to the task or job, serve to facilitate organizational functioning” (Lee and Allen, 2002, p 132)

Lambert, S.J.  (2006) Both art and science: Employing organizational documentation in workplace-based research.  In Pitt-Catsouphes, M., Kossek, E.E., & Sweet, S. (Eds.). The work and family handbook: Multi-disciplinary perspectives, methods, and approaches.  (pp.503-525).  Mahwah, New Jersey: Lawrence Erlbaum Associates.   Lee, K., & Allen, N. J.  (2002). Organizational citizenship behavior and workplace deviance: The role of affect and cognitions.  Journal of Applied Psychology, 87(1), 131-142.