Definition: Tautology

According to wikipedia.org, “In logic, a tautology is a formula or assertion that is true in every possible interpretation.”

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 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 that 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.