Sentence:  The international unit ( u . ) , adopted to make possible the comparison of results from different laboratories ( Mussett and Perry , 1955 ) , has been defined as the <WSD>amount</WSD> of activity present in 13.5 mg of the International Standard Preparation . 
Meaning of the word marked with <WSD> tags:  how much there is or how many there are of something that you can quantify