What is this type of mutation best described?

The answer to this is Deletion.

DNA deletion is the process wherein a part of the chromosome has been removed during the DNA replication. There are times when a part of the DNA is not copied properly. There are some diseases that may be stemmed from the DNA deletion such as cystic fibrosis, Turner Syndrome, William Syndrome, and so much more.

Some people have speculated whether this is hereditary or not bur studies show that it isn’t. Each cell is responsible for its own replication. If in case deletion does happen, it is because of the cell and not because of the family genetics.