Sentence:  The collection of all possible unions of basis elements of a <WSD>basis</WSD> is said to be the topology generated by that basis. 
Meaning of the word marked with <WSD> tags:  A collection of subsets ("basis elements") of a set, such that this collection covers the set, and for any two basis elements which both contain an element of the set, there is a third basis element contained in the intersection of the first two, which also contains that element.