Tautology
In logic, a tautology is a formula or assertion that is true in every possible interpretation.
https://en.wikipedia.org/wiki/Tautology_(logic)
What it basically means
Always true
What’s the difference
Tautology gives a little more nuance to the phrase in that it’s always true “under every interpretation”.
Wiki does give a really good example of tautology:
An example of a tautology is “(x equals y) or (x does not equal y)”. A less abstract example is “The ball is green, or the ball is not green”. It is either one or the other—it cannot be both and there are no other possibilities.
Techopedia describes a tautology in a computing point of view as redundant logic which could accidentally happen when coding up an expression.
So if someone says that an expression is a tautology, they just mean that it always evaluates as being true.