Xenobiology (sometimes referred to as astrobiology) is the study of biology of foreign life forms. The Anglic prefix xeno- means "foreign", so the term refers to the biological science of life forms not native to the world of the scientist.
Within the Third Imperium the term refers to Comparative Xenobiology, comparing how lifeforms evolved and exist in different environments, parallel evolution, and how life evolved in different worlds interact when placed in the same biosphere.
There is a related term of Exobiology, where the prefix Exo- means outer or external. In some cases this becomes a synonym for Xenobiology, but in modern use Exobiology refers to the study of things outside of normal biology including Unconventional Life and Exotic Life.