Quintessence is a term that originates from ancient philosophy, referring to the fifth element that complements the four classical elements: earth, water, air, and fire. In this context, quintessence represents the essence of all matter and is thought to be the purest form of substance, often associated with the heavens or celestial bodies. In modern cosmology, quintessence describes a hypothetical form of dark energy that could explain the accelerated expansion of the universe. Unlike the cosmological constant, quintessence is dynamic and can change over time, potentially influencing the universe's fate and structure.