An autosome is a chromosome that is not an allosome (i.e., not a sex chromosome). Autosomes appear in pairs whose members have the same form but differ from other pairs in a diploid cell, whereas members of an allosome pair may differ from one another and thereby determine sex. The DNA in autosomes is collectively known as atDNA or auDNA.
For example, humans have a diploid genome that usually contains 22 pairs of autosomes and one allosome pair (46 chromosomes total). The autosome pairs are labeled with numbers (1-22 in humans) roughly in order of their sizes in base pairs, while allosomes are labeled with their letters. By contrast, the allosome pair consists of two X chromosomes in females or one X and one Y chromosome in males. (Unusual combinations of XYY, XXY, XXX, XXXX, XXXXX or XXYY, among other allosome combinations, are known to occur and usually cause developmental abnormalities.)