The expression "18 less than a number" can be translated into a mathematical expression as:

$x - 18$

where $x$ represents the number.

Would you like further details or have any questions?

Here are 5 related questions to explore:

1. How would you translate "20 more than a number" into a mathematical expression?
2. If the number $x$ is 25, what is "18 less than that number"?
3. What does the expression $x - 10$ represent in words?
4. How do you translate "a number decreased by 5" into a mathematical expression?
5. Can you solve for $x$ if $x - 18 = 12$?

Tip: Phrases like "less than" indicate subtraction, and the number being subtracted usually comes after the variable in the expression.