partitive

Concise Oxford English Dictionary © 2008 Oxford University Press:
partitive /ˈpɑːtɪtɪv/ Grammar

▶adjective denoting a grammatical construction indicating that only a part of a whole is referred to, for example a slice of bacon, some of the children. ▶noun a noun or pronoun used as the first term in such a construction.

– derivatives
partitively adverb.