Sentence:  The variation in the 3 - cm emission of the moon during a lunation is very much less than the variation in the 8.6 - <WSD>mm</WSD> emission , as would be expected from the explanation of Piddington and Minnett ( 1949 ) . 
Meaning of the word marked with <WSD> tags:  a metric unit of length equal to one thousandth of a meter