Here's why:
* Expression: A mathematical expression is a combination of numbers, variables, and operations (like addition, subtraction, multiplication, division) that doesn't include an equal sign. "8z 5 18" fits this definition.
* Equation: An equation includes an equal sign, showing a relationship between two expressions. "8z 5 18" is not an equation.
* Function: A function is a specific type of relationship between inputs and outputs, often written with an equation. "8z 5 18" doesn't define a function.
* Variable: A variable is a symbol that represents a quantity that can vary. "z" is a variable in this expression, but the expression itself is not a variable.
Therefore, expression is the most accurate term to describe "8z 5 18."