1) A double membrane enclosing a cell nucleus and having its outer part continuous with the endoplasmic reticulum -- called also karyotheca.

2) A nuclear membrane is a double membrane that encloses the cell nucleus. It serves to separate the chromosomes from the rest of the cell. The nuclear membrane includes an array of small holes or pores that permit the passage of certain materials, such as nucleic acids and proteins, between the nucleus and cytoplasm.