What does the ordinal number of an element mean?

The ordinal number of an element or its proton number means:
– the charge of the nucleus of an atom of an element;
– the number of protons (positively charged particles) in the nucleus;
– the number of electrons (negatively charged particles) in an atom.
The number of protons and electrons in an atom is the same, which is why an atom is an electrically neutral particle.
For example, Ca (calcium) has serial number 20 (see the periodic table), which means the charge of its nucleus is +20, because there are 20 protons in the nucleus, and 20 electrons revolve around the nucleus.