Which word represents a linguistic phenomenon where a proper name becomes generic?

The correct answer is "Commonisation," which refers to the process by which a proper noun evolves into a common noun, losing its original specific reference and becoming a general term for a class of objects or concepts. This often occurs when a brand name or a specific product name becomes so widely recognized that it is used to denote similar items in general.

For instance, the brand name "Kleenex," originally a trademark for a specific type of tissue, is now frequently used to refer to any facial tissue regardless of the brand. This phenomenon illustrates how language adapts and changes over time, as certain names enter everyday vocabulary, demonstrating a shift from specific to general reference.