Use this tag for questions on the concept of infinity. Don't use this tag merely because $\infty$ appears somewhere in the question.
A lay person might think they know what infinity is, but a mathematician will ask you "what kind of infinity are you talking about, and in what context?"
Good questions about Infinity will need to be explicit about the context in which the question is being asked; many of the questions already tagged Infinity essentially have the answer "The answer is undefined, because infinity isn't an ordinary number" or "Your question can't be answered without giving more information." And there is probably already a different tag covering whatever field of mathematics the question is about.
Questions about fallacies involving infinity are on-topic for this tag.