nucleon: Either a proton or a neutron; the term neucleon is used when one is not concerned with distinguishing between protons and neutrons, and rather is refering to either usually as a resident of the nucleus.
nucleus: The center --- usually of the central piece of an atom. The nucleus of an atom is composed of protons and neutrons, which are themselves composed of quarks. The nucleus of an atom is about 1000 to 10,000 times smaller than the size of an atom, where the size of the atom is defined by the orbit of the electrons.
numerical calculations: calculations done using numbers, as opposed to symbolically; typically on a computer.