General definition (courtesy of the wiki): A fixed point (sometimes shortened to fixpoint, also known as an invariant point) is a value that does not change under a given transformation.

So x is a fixed point of a function f if f(x) = x. This can be generalized to various kinds of things in mathematics, but as an algebraic example:

  f(x) = x^2

1 and 0 are fixed points:

  f(1) = 1^2 = 1
  f(0) = 0^2 = 0

https://en.wikipedia.org/wiki/Fixed_point_(mathematics)