In general, a nucleus is the core or the central part around which other parts are grouped or gathered.
In Biology, a nucleus is the large, membrane-bounded organelle that contains the genetic material, in the form of multiple linear DNA molecules organized into structures called chromosomes. Its major functions are to maintain the integrity of DNA and to control cellular activities such as metabolism, growth, and reproduction by regulating gene expression. It has three main components: the nucleolus, the chromatin and the nuclear envelope. The nuclear envelope has nuclear pores to control the movement of molecules between the nucleoplasm and the cytoplasm.
In other fields, such as: