1. The problem asks: What is the term used to describe $x$ in an expression? 2. In algebra, an expression is made up of terms, which can be constants, variables, or products of constants and variables. 3. The symbol $x$ is typically called a "variable" because it can represent different values. 4. Variables are placeholders that can take on various numerical values in expressions or equations. 5. For example, in the expression $3x + 5$, $x$ is the variable, $3$ is the coefficient, and $5$ is a constant term. 6. Therefore, the answer is: $x$ is called a variable.