Nucleon is a generic word for the heavy particles that make up the atomic nucleus: the protons and neutrons. It is like the generic word fruit, used to include apples, oranges, and many others, except that the class of nucleons contains only two members.
The nucleon number of a nucleus is just another term for mass number; it is simply the total number of nucleons in the nucleus. In the symbol for a particular kind of nucleus, the nucleon number or mass number is written to the upper left of the symbol of the element. For example, the symbol for a nucleus of carbon-14, the isotope of carbon that contains six protons (because all carbon nuclei do) plus eight neutrons for a total of 14 nucleons, is 14C.
The use of the ending -on for the names of subatomic particles began with electron, a word that was coined in 1891 by the Irish physicist George J. Stoney (1826-1911) by modifying the word electric to come up with a name for the basic unit of electricity. This was six years before J. J. Thomson (1856-1940) actually measured the electron as a particle.
After Ernest Rutherford (1871-1937) discovered the atomic nucleus in 1911, he proposed the name proton for the very lightest of all nuclei: the nucleus of the ordinary hydrogen atom. Proto- is Greek for first. In 1932, when James Chadwick (1891-1974) discovered another particle in the nucleus that was very similar to the positive proton except that it was electrically neutral, it was natural for him to call it a neutron. It was then equally natural to call both nuclear particles nucleons, especially when nuclear theory began to treat the proton and the neutron as two different states of the same fundamental particle.
Nucleons are no longer thought to be the ultimate nuclear particles, however. Current theory says that each proton and neutron is made up of a trio of fundamental particles called quarks—particles that are thought to be the truly basic entities in nuclear matter. The proton is thought to consist of two quarks of charge +2/3 each (called up quarks) and one quark of charge -1/3 (called down quarks). The charge of the whole proton, which is +1, is the sum of the charges of its quarks: two times +2/3 and one times -1/3 make +3/3 or +1. The neutron, on the other hand, consists of one up quark of charge +2/3 and two down quarks of charge -1/3 each. The neutron's net charge of zero comes from the charges of its quarks: one times +2/3 and two times -1/3 make zero.