The symbol $i$ represents the imaginary unit in mathematics, which is defined as the square root of $-1$. In other words:

$i = \sqrt{-1}$

This means that $i^2 = -1$. The imaginary unit is a fundamental component of complex numbers, which are expressed in the form $a + bi$, where $a$ and $b$ are real numbers.

Would you like more details on how complex numbers work or how they are used? Here are some related questions that might interest you:

1. What are complex numbers, and how are they different from real numbers?
2. How do you add and subtract complex numbers?
3. What is the geometric interpretation of complex numbers on the complex plane?
4. How do you multiply and divide complex numbers?
5. What are the applications of complex numbers in engineering and physics?
6. How does Euler's formula relate to complex numbers?
7. What is the significance of the complex conjugate?
8. How do you find the magnitude and argument of a complex number?

Tip: Understanding the imaginary unit $i$ is crucial for grasping the concept of complex numbers and their applications in various fields, including engineering, physics, and mathematics.