Loss of electrons is called oxidation.

Explanation:
Oxidation is a process in which a substance loses electrons. It is characterized by an increase in the oxidation state of an atom, ion, or molecule. The substance that undergoes oxidation is called the reducing agent because it causes another substance to be reduced by accepting the electrons that are lost.

Example:
Let's consider the following reaction:
2Na + Cl2 → 2NaCl

In this reaction, sodium (Na) is oxidized because it loses an electron and its oxidation state increases from 0 to +1. Chlorine (Cl2) is reduced because it gains an electron and its oxidation state decreases from 0 to -1. Therefore, sodium acts as the reducing agent, and chlorine acts as the oxidizing agent.

Rules for determining oxidation state:
- In a neutral atom, the oxidation state is zero.
- The sum of oxidation states in a compound is zero.
- The oxidation state of an element in its elemental form is zero.
- The oxidation state of oxygen in most compounds is -2.
- The oxidation state of hydrogen in most compounds is +1.

Importance of oxidation:
Oxidation is a fundamental process in various chemical reactions. It plays a crucial role in energy production, such as in cellular respiration and combustion reactions. Oxidation reactions are also essential for the extraction of metals from their ores and the rusting of iron. In addition, oxidation reactions are involved in many biological processes, such as metabolism and the immune response.

Summary:
Loss of electrons is called oxidation. It is a process in which a substance loses electrons and its oxidation state increases. Oxidation reactions are important in various chemical and biological processes.