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