Final answer:
The example provided where Billy corrected his friend's use of 'ophthalmologist' by saying 'eye doctor' is an example of synonymy.
Step-by-step explanation:
The example provided where Billy corrected his friend's use of 'ophthalmologist' by saying 'eye doctor' is an example of synonymy. Synonymy refers to using different words or phrases to express the same meaning. In this case, 'ophthalmologist' and 'eye doctor' are synonyms, both referring to a medical professional who specializes in diagnosing and treating eye diseases and disorders.