In various contextual positions one member of an opposition can be used in the position of the other. This phenomenon can be referred to as reduction of oppositions.
A stylistically indifferent oppositional reduction is called neutralization. Use of the unmarked member does not transgress the expressive conventions of ordinary speech.
Another type of oppositional reduction is called transposition. It is defined as contrastive use of the counter-member of the opposition (the strong one, as a rule).