What is the mutation caused by the addition of a nucleotide to an already existing gene sequence called?

Insertion is a type of mutation which adds nucleotide to an already existing gene sequence. It can lead to genetic disorders due to the change of gene sequence. An example of this genetic disorder is the trisomy 21 or commonly known as Down syndrome. Instead of having 46 cells, those people having this disorder have 47 cells because there is an extra copy of there gene 21.