Trisomy 21

 Trisomy 21 is the name given to the most common occuring mutation that leads to Down's syndrome. "Trisomy" because, due to a nondisjunction event (failed separation of the chromosome), there is an extra copy of chromosome 21 in the gamete. And "21" states that the trisomy occurs on chromosome 21. 

The non-disjuntion event describes the failure of the chromosome pairs to separate properly during meiosis, either stage 1 or stage 2, such that one of the gametes will contain 1 chromosome 21 and one will contain 1 more, the latter being the trisomy.