system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can a mathematician use 'infinity' in calculations and what types of 'problems' require them to do so?
gpt
Infinity is a concept, not an actual number. It usually refers to something that isn't finite, has no end, or is unbounded. The actual meaning of infinity depends on the context, and whenever the term infinity is used, it needs to be accompanied by a proper, strict definition of what this infinity stands for. 

For example: in calculus, infinity relates to limits - we say that a function's limit at a certain point is infinity if the value of the function keeps rising and rising the closer we get to that point. There is an actual, strict definition of what it means to say the limit of the function is infinity.

In set theory, however, infinity relates to sizes of sets. As such, there are different kinds of infinities, since two infinite sets can have different sizes.