An expression is a unit of code that can produce a value. You can use the word expression to mean a statement, or a sub-unit of code within a statement. For example, if a function requires a numeric value as a parameter, you may provide any of:
• a number (5)
• a variable that holds a numeric value (x)
• another function that returns a numeric value: sqrt(25)
• a calculation that returns a numeric value: (3 + 5 / 2 ).
Rather than list all four options, we would simply say that the parameter must be "any expression that returns a numeric value". You will see the word "expression" used most often in this context.