1. A set of forms having a common root or stem , of which one form must be selected in certain grammatical environments.
2. The set of substitutional relationships a linguistic unit has with other units in a specific context.
Payne, Thomas E. (1997) .Describing morphosyntax: A guide for field linguists. Cambridge; New York: Cambridge University Press.
Crystal, David. 1997.A dictionary of linguistics and phonetics. 4th edition. Cambridge, MA: Blackwell.