An expression is a finite collection of symbols appropriately written so that they make sense. An expression can be thought of as one side of an equation and can include variables, operators or constants. Some examples of expressions are as follows.

• $ax^2+bx+c$
• $\int_{-\infty}^x\frac{e^t}{t}\text{d}t$
The following, however, is not an expression as it doesn't make sense (isn't well formed) according convention.

• $x(^n\frac{x}{\text{d}x^2}\int[$