1. The hind-part of the body of an arachnid, consisting in the most primitive forms of thirteen segments and a telson, but tending to become shortened and very variable in the majority of the more specialized forms.

In spiders, it forms a more or less globular and apparently unsegmented "abdomen". In scorpions, it forms a segmented tail with a sting at the end.

2. The posterior or back part of an arachnid's body, behind the prosoma (cephalothorax or head).

Although it is similar in most aspects to an abdomen (and is often referred to as such), the opisthosoma is differentiated by its inclusion of the respiratory organs and the heart. The number of segments and appendages on the opisthosoma vary.