Demotion

Demotion is the phenomenon that a subject (or external argument) becomes an optional oblique phrase or adjunct due to some morphological or syntactic operation.

Example

if we form the passive of break (= broken), the subject (or external argument) is demoted to become an adjunct (= the by-phrase):

(i)  Tom broke the vase
(ii) The vase was broken (by Tom)

Link

Utrecht Lexicon of Linguistics

References

• Marantz, A. 1984. On the Nature of Grammatical Relations, Cambridge, Mass., MIT Press.
• Spencer, A. 1991. Morphological Theory, Blackwell, Oxford.
• Williams, E. 1981b. Argument Structure and Morphology, The Linguistic Review 1, pp. 81-114