Sentence:  Every number has a <WSD>digit sum</WSD>. The <WSD>digit sum</WSD> of 7 is seven. The <WSD>digit sum</WSD> of 62 is eight (6 + 2). The <WSD>digit sum</WSD> of 728 is also eight (7 + 2 + 8=17, 1 + 7 = 8). The digit sum of a number is found by adding all the digits in the number until a single digit is left. 
Meaning of the word marked with <WSD> tags:  the number obtained when repeatedly applying above described operation until arriving at a single-digit number.