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?"
See the question Is Infinity a number and in particular this answer for a good discussion on a number of things that people sometimes mean when they talk about "infinity" in mathematical terms.
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.
Please check the questions What is the result of $\infty-1$? and What is the result of infinity minus infinity? before asking questions about doing arithmetic on infinity.
Questions about fallacies involving infinity are on-topic for this tag.
