**Definition:**
The real number system consists of all the numbers that can be found on the number line, including both rational and irrational numbers.

**Key ideas:**
- Rational numbers can be expressed as fractions, such as 1/2 or -3.
- Irrational numbers cannot be written as simple fractions, like √2 or π.
- Real numbers include whole numbers, integers, and decimals.
- The real number line is continuous, meaning there are infinitely many numbers between any two numbers.

**Why this topic matters in this category:**
Understanding real numbers is essential for solving mathematical problems and applying concepts in various scientific fields.