Final answer:
Acquired canities is the term used to describe the loss of hair pigment as a person ages, leading to gray or white hair. This happens due to a decrease in melanin production by melanocytes in the hair papilla.
Step-by-step explanation:
The loss of pigment in the hair as a person ages is called acquired canities. This condition occurs because melanin production decreases, which leads to hair losing its color, becoming gray or white. The pigment melanin is produced by melanocytes located in the hair papilla and is responsible for giving hair its color. The amount and type of melanin, which are genetically determined, define the hair color. As individuals age, the activity of these melanocytes diminishes, leading to less pigment in the hair.