In linguistics, the partitive is a word, phrase, or case that divides something into parts. For example, in the English sentence I'll have some coffee, some is a partitive determiner, because it makes the noun phrase some coffee refer to a subset of all coffee. Similarly, the preposition of often serves as a partitive, as in many of my friends, the youngest of the children, a glass of wine, some of the milk, and some of the people.
In English, the use of the partitive some is optional: I'll have some coffee has very nearly the same meaning as I'll have coffee. In most Romance languages, however, the partitive must be included. For example, in French, I want to drink some coffee is expressed as Je veux boire du café; here du (a contraction of de le "of the") is the partitive and is the equivalent of the optional English some. The feminine form is de la, and the plural form is des (a contraction of de les "of the (plural)").
Some languages, for example Estonian and Finnish, have a special partitive case. In Latin, German and Russian, the partitive is expressed by the genitive case. For a limited number of words in Russian, though, a partitive expression has its own separate formulation; this group is generally regarded as a special type of the genitive (also called the "second genitive") case. Its use is quite common.