Why is losing electrons called oxidation?

Hint: Electrons are one of the sub-atomic particles present in an atom. When any light energy or chemical reagents were attacked on the atoms, electrons may lose and form a positive ion known as a cation. The definition of oxidation can be described in the following three ways.

Complete answer:
An atom is a stable tiny particle consisting of three subatomic particles namely electrons, protons, and neutrons. When an atom loses electrons, then the atom can be said to be oxidized and the process of losing electrons is known as oxidation.
First, the addition of oxygen atoms is defined as oxidation. Let us consider an example of a chemical reaction that involves the addition of oxygen as follows:
 $ 2Mg + {O_2} \to 2MgO $
In the above reaction, magnesium reacts with oxygen and forms a compound known as magnesium oxide. In this reaction, the oxygen atoms were gained by the magnesium atom.
At first, magnesium and oxygen are at zero oxidation states. In the magnesium oxide, magnesium is in $ + 2 $ oxidation state and oxygen is in $ - 2 $ oxidation state. Thus, it is clear that magnesium undergoes a loss of electrons.
Thus, when there is an addition of an oxygen atom to any atom it leads to the loss of electrons in that atom or compound makes the definition of oxidation as loss of electrons.

Note:
Oxidation can be defined in three ways like the addition of oxygen atoms, loss of electrons, and loss of hydrogen atoms. Chemists added these three definitions of oxidation to explain both the organic and inorganic concepts.