Sentence:  Hogarth's enlargement of painting's scope was based on the <WSD>conversation piece</WSD>, which was to prove one of the most flexible and adaptable of genres. The idea of  an informal group was not in itself new, nor the invention of Hogarth alone. 
Meaning of the word marked with <WSD> tags:  A painting of a group of people engaged in conversation or some other activity, popular in 18th century.