Yellowing refers to the process by which an object or substance gradually turns yellow over time. There are many synonyms for this term, including discoloration, fading, browning, and tarnishing. Discoloration is a broad term that refers to any change in color, regardless of the specific hue. Fading typically refers to the gradual loss of color intensity, while browning specifically refers to the process by which something turns brown. Tarnishing is a term typically used in reference to metals, and describes the process by which they develop a patina over time. Regardless of the specific term used, yellowing is a natural process that occurs because of exposure to light, air, and other environmental factors over time.