An autosome is a non-sex chromosome. It is an ordinary paired chromosome that is the same in both sexes of a species. For example, in humans, there are 22 pairs of autosomes. The X and Y chromosomes are not autosomal.
Non-autosomal chromosomes are usually referred to as sex chromosomes or, less frequently, as gonosomes.
An autosomal dominant gene is one on an autosome and is always expressed, even if a single copy exists. The chance is 1:2 for passing this gene to offspring.