What is the elementary charge?

Hint: The elementary charge is the fundamental value of the charge that can be possessed by existing charged particles. It is basically the magnitude of the charge on an electron. Other particles possess charge in some multiples of the elementary charge.
\[\text{1 elementary charge (e)}=1.602176487\times {{10}^{-19}}\text{C}\]

Complete answer:
Similar to time, length, or mass, the elementary charge is a fundamental measurement of charge possessed by an entity. The elementary charge is the smallest electric charge possible that was discovered way before the discovery of quarks. It can be defined as the magnitude of the electric charge carried by a single electron or proton. According to the International System of Units, it is written in the units of coulomb.
The numerical value of the elementary charge is approximately equal to $1.602176487\times {{10}^{-19}}$ coulombs. And it is denoted by the symbol ‘e’.
The elementary charge is also the electrical charge carried by a proton, however with opposite polarity. In other words, protons have a positive (+e) charge, whereas electrons have a negative (-e) charge.
The importance of the elementary charge lies in the fact that almost all existing charged subatomic matter discovered so far, including electrons, protons, positrons, etc, have either electric charge equal to the value of the elementary charge or can be expressed by whole number multiples of the value.
The elementary charge was first observed and measured by Robert Millikan in 1909 using his famous oil-drop experiment.

Note:
The neutron is an electrically neutral subatomic particle and it is said to possess no charge on it. However, the value of charge on neutron in terms of the elementary charge can be expressed as follows:
\[\text{Charge on neutron}=0\times \text{e}=0\]
Where e is the elementary charge.