The **ESU** (Electrostatic Unit) is part of the **CGS (centimeter-gram-second)** system of units, not the **SI (International System of Units)**.
In the CGS system, the ESU is used to measure quantities related to electrostatics, such as electric charge. Specifically, the **electrostatic unit of charge** (often called **esu of charge**) is used in the CGS system to quantify charge in a way that is different from the Coulomb in the SI system.
In SI, the unit of electric charge is the **Coulomb (C)**, while in CGS, it's the **esu of charge**. These two units are related, but they are used in different unit systems.
So, to summarize:
- **ESU** is a CGS unit.
- In the **SI system**, the corresponding unit of charge is the **Coulomb**.
Would you like to know how these units are related or how to convert between them?