The nuclear envelope is a defining feature of eukaryotic cells. It is composed of two lipid bilayer membranes separated by a narrow perinuclear space. Within the envelope, the nuclear pores regulate the movement of molecules in and out of the nucleus. Synonyms for the nuclear envelope include nuclear membrane, nucleolemma, and karyotheca. These terms are often used interchangeably to describe this crucial structure, which protects the DNA inside the nucleus. Understanding the nuclear envelope is essential for studying cellular processes such as gene expression, DNA replication, and cell division. As research into these areas continues, scientists are likely to develop new ways of describing and investigating the nuclear envelope and its functions.