What is meant by "commonisation" in linguistics?

Commonisation refers specifically to the linguistic process where a proper name transitions into a general term. This can occur when a name that originally pertains to something specific becomes used to describe a broader category. For example, "thermos" originally referred to a brand of vacuum flask but has since come to be used generically to describe similar vacuum containers. This illustrates how commonisation enables distinctive names to evolve into terms that are widely understood and used across various contexts.

The other options highlight different linguistic processes but do not define commonisation correctly. For instance, narrowing refers to a word's meaning becoming more specific, while broadening involves an expansion of meaning, and pronunciation changes relate to phonetics rather than semantic evolution.