Final answer:
In the reaction notation '68Zn (p, 2n) 67Ga', '68Zn' is the target nucleus that is being bombarded with a proton to form the new nucleus, Gallium-67, and emitting two neutrons in the process.
Step-by-step explanation:
In the nuclear reaction notation 68Zn (p, 2n) 67Ga, 68Zn is called the target. This is because it is the nucleus which is targeted by the proton (indicated by 'p') in the reaction to produce another nucleus, in this case, Gallium-67 (67Ga) through the emission of two neutrons (indicated by '2n').
68Zn is not a proton, neutron, or an isotope in this context—it refers to an isotope of the element Zinc (Zn) that has an atomic number of 30 (the number of protons) and a mass number of 68 (the total number of protons and neutrons). In a nuclear reaction, the starting nucleus like 68Zn is generally referred to as the target or sometimes as the nuclide.