A function is "even" when
f(x) = f(−x) for all x
In other words there is symmetry about the y-axis (like a reflection):
This is the curve f(x) = x2+1
They got called "even" functions because the functions x2, x4, x6, x8, etc behave like that, but there are other functions that behave like that too, such as cos(x)
Cosine function: f(x) = cos(x)
It is an even function
But an even exponent does not always make an even function, for example (x+1)2 is not an even function