Sentence:  Like the em unit, the <WSD>rem</WSD> unit is based on declared font size. The difference—and it's a doozy—is that whereas em is calculated using the font size of the element to which it's applied, <WSD>rem</WSD> is always calculated using the root element. 
Meaning of the word marked with <WSD> tags:  A unit relative to the declared font size of the root element in a HTML document.