What is the primary distinguishing feature of eukaryotic organisms?

The primary distinguishing feature of eukaryotic organisms is the presence of a nucleus. In eukaryotic cells, the nucleus serves as a membrane-bound compartment that houses the cell's genetic material (DNA). This organization allows for complex regulation of gene expression and processes such as replication and transcription, which are more compartmentalized compared to prokaryotic cells, where the genetic material is not enclosed within a membrane.

While some other characteristics, such as the presence of organelles or the ability to be multicellular, can also be associated with eukaryotes, it is the nucleus that fundamentally differentiates them from prokaryotic organisms. Prokaryotes, which include bacteria and archaea, lack a nucleus, and their DNA floats freely within the cell. This fundamental difference in cellular organization is the cornerstone of the distinction between these two major domains of life.