Polytextuality is a measure of the degree of independence of a linguistic unit from its context (or co-text).
This concepts can be oprationalised, depending on the unit under study, in different ways. The P. of a word is usually measured by determining the number of different texts in a corpus, in which the given word occurs at least once (in informations retrieval, this measure is nowadays called "document frequency", which might be considered as a rather misleading term). The P. of a morpheme might be measured in terms of the number of different words it occurs in (by some authors confused with the term "productivity").
Word P. morpheme P. and other units have been investigated with respect to their distributions and to their interrelations to some other properties of these units.
Köhler, Reinhard (1986): Zur linguistischen Synergetik. Struktur und Dynamik der Lexik. Bochum: Brockmeyer.