'An individual's belief regarding the terms and conditions of a reciprocal exchange agreement between that focal person and another party … a belief that some form of promise has been made and that the terms and conditions of the contract have been accepted by both parties'

from Robinson, S.L. & Rousseau, D.M. (1994). 'Violating the psychological contract: Not the exception but the norm' Journal of Organizational Behavior 15, pp. 245-259.