Nuclear means "of or belonging to the nucleus (or center) of something". Which pronunciations of "nuclear" are commonly acceptable is a matter of some debate and amusement; see Nucular for an example.
Where the subject is the nucleus of an atom, "nuclear" usually refers to nuclear physics.
The term nuclear is also sometimes used to refer to a family type. See: nuclear family.
This is a disambiguation page; that is, one that just points to other pages that might otherwise have the same name. If you followed a link here, you might want to go back and fix that link to point to the appropriate specific page.