A mathematical expression is a combination of numbers, variables, and operators that represents a value. For example, in the expression 3x + 5, 3 is a coefficient, x is a variable, and 5 is a constant. Expressions can be simple, like 2 + 3, or more complex, involving multiple operations and variables. Mathematical expressions do not include an equality sign, which distinguishes them from equations. They can be evaluated to find a specific value when the variables are replaced with numbers. Understanding expressions is fundamental in fields like algebra and calculus, where they are used to model real-world situations.