A function $f$ is an even function if for all $x$ in the domain of $f$,

$f(-x) = f(x).$

Would you like more details or have any questions?

Here are 5 related questions you might want to ask:

1. What is an odd function?
2. Can you provide examples of even functions?
3. How do you determine if a function is even or odd?
4. What are the properties of even functions?
5. Can a function be both even and odd?

Tip: For a function to be even, its graph must be symmetric with respect to the y-axis.