Sentence:  We say that a set S has a "finite partition" into <WSD>subsets</WSD> S_1,·s,S_n, if S = S_i \cup·s\cup S_n, where the <WSD>subsets</WSD> are pairwise disjoint, that is, S_i\cap S_j = \empty, if i\ne j. (We do not require that the <WSD>subsets</WSD> be nonempty.) 
Meaning of the word marked with <WSD> tags:  A set "A" such that every element of "A" is also an element of "S".