Sentence:  A <WSD>Porism</WSD> is a proposition in which it is proposed to demonstrate that some one thing, or more things than one, are given, to which, as also to each of innumerable other things, not given indeed, but which have the same relation to those which are given, it is to be shewn that there belongs some common affection described in the proposition. 
Meaning of the word marked with <WSD> tags:  A corollary.