a grammatical construction which conforms to the meaning rather than to strict syntactic agreement or reference (Ex.: Has everyone washed their hands?)
See synesis in American Heritage Dictionary 4
syn·e·sis
noun
A construction in which a form, such as a pronoun, differs in number but agrees in meaning with the word governing it, as in If the group becomes too large, we can split them in two.
Origin: Greek sunesis, union, understanding, from sunīenai, to understand, bring together : sun-, syn- + hīenai, to send, hurl; see yē- in Indo-European roots.