Sentence:  But poverty's scourge is fiercest below $1.25 (the average of the 15 poorest countries' own poverty lines, measured in 2005 dollars and adjusted for <WSD>differences</WSD> in purchasing power): people below that level live lives that are poor, nasty, brutish and short. 
Meaning of the word marked with <WSD> tags:  A characteristic of something that makes it different from something else.