Yellowing - What is yellowing?

Definition of YELLOWING:

A defect in the appearance of paper or other materials characterised by a gradual change in colour, most commonly involving yellow or brown hues forming. Yellowing may occur when a label is exposed to sunlight or UV light for extended periods of time; it is also a natural effect of the passage of time.

Here is the Harvard-style citation to use if you would like to reference this definition of the word yellowing:
Label Planet (2020) What is yellowing? | Yellowing Definition. Available at: https://www.labelplanet.co.uk/glossary/yellowing/ (Accessed: January 1, 2023).