"Spontaneous mutation" refers to a random change that occurs in the DNA of an organism without any external cause or influence. It can happen naturally and unexpectedly, leading to a new characteristic or trait. Full definition
In addition, they identified 14 new developmental disorders, all caused by spontaneous mutations not found in either parent. (sciencedaily.com)
Tests showed that the DNA of the cells was damaged, but that the mutations were smaller than would be expected with a nuclear hit — resembling spontaneous mutations. (sciencemag.org)
Most spontaneous mutations do neither harm nor good but simply accumulate in the genome, one at a time, as they are passed from one generation to the next. (discovermagazine.com)