Modal interchange · Music theory
Borrowing chords from the minor mode, or from other modes, without changing the tonic; which they are and what function they have.
Borrowing without changing the tonic
A key and its parallel share the tonic, so they can lend each other chords: in C major you can use chords from C minor without leaving C. That mixture is called modal interchange. It darkens the color for a moment, without losing the center: another way of feeling the same key.
The classical tradition already did this: in Schubert or Brahms, the alternation of major and minor on the same tonic is an expressive signature. And the Picardy third, seen in the minor scales, is the opposite gesture: a major chord borrowed to end in minor.