Sentence:  Given an integer X written in base 10 (as is usual), compute the sum of the digits of X: call the result the <WSD>digit sum</WSD> of X. If the <WSD>digit sum</WSD> is greater than 9, we form the <WSD>digit sum</WSD> again. Continue in this way to obtain the iterated <WSD>digit sum</WSD> which is at most 9. 
Meaning of the word marked with <WSD> tags:  The sum of all digits.