In jazz improvisation, a substitution refers to the practice of replacing one chord or harmonic structure with another while maintaining the overall harmonic function or progression of the original composition. Substitutions can involve replacing entire chords or altering specific chord tones to create new harmonic colors or tensions. This technique allows improvising musicians to add variety, complexity, and personal expression to their solos while still fitting within the framework of the original chord progression. Substitutions can range from simple alterations, such as replacing a major chord with its relative minor, to more complex reharmonizations involving advanced harmonic concepts.