syn·e·sis 
(sĭnĭ-sĭs)
(sĭnĭ-sĭs)n.
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.
[Greek sunesis, union, understanding, from sunīenai, to understand, bring together : sun-, syn- + hīenai, to send, hurl; see yē- in the Appendix of Indo-European roots.]