A domain is the highest level in taxonomy, the science of naming and classifying groups of organisms based on similarities.

There are three domains:
Eukaryota, organisms whose cells contain a membrane-bound nucleus containing chromosomal DNA;
Bacteria, single-celled organisms with no nucleus and with a single circular DNA chromosome in the cytoplasm;
Archaea, also single-celled organisms with no nucleus and with a single circular DNA chromosome in the cytoplasm, but with genes and metabolic pathways more like those of Eukaryota