Definition of "bleaching"

Grammaticalization is often associated with 'semantic bleaching', and this 'bleaching' is the result of reanalysis […] But there is no evidence that the bleaching of the meaning of do played any role in the causation of this sequence of events.

2000, Frederick J. Newmeyer, Language Form and Language Function, MIT Press, page 249

In the development of the prepositional phrase, which did not exist in PIE, one can see a grammaticalization based on a double bleaching. […] Many of these combinations are achieved by lexical bleaching of the grammatical element and grammatical bleaching of the lexical element, as in the prepositional phrases in (11).(11) French English en auto by car

2009, Vit Bubenik, John Hewson, Sarah Rose, Grammatical Change in Indo-European Languages: Papers presented at the workshop on Indo-European Linguistics at the XVIIIth International Conference on Historical Linguistics, Montreal, 2007, John Benjamins Publishing, page 165