**Definition:**
This node focuses on the concept of functions, which are mathematical relationships where each input has a unique output. Functions are essential for understanding how different quantities relate to one another.

**Key ideas:**
- A function is often written as f(x), where x is the input.
- Functions can be linear, quadratic, or exponential, each with different characteristics.
- The graph of a function visually represents the relationship between inputs and outputs.
- Understanding functions is crucial for solving equations and real-world problems.

**Why this topic matters in this category:**
Functions are foundational in mathematics, helping students analyze and model various scientific and practical situations.