What is the extensive use of note from outside the prevailing key signature?

The extensive use of notes from outside the prevailing key signature is known as modal interchange or modal mixture. This technique involves borrowing chords or notes from parallel modes (e.g., using notes from C minor while in C major) to create contrast and emotional depth in music. It adds richness and complexity, allowing composers to explore a wider range of harmonic and melodic possibilities. This practice is common in various genres, enhancing the expressive quality of a piece.