Inspired by this q/a and the comments on it.
I'm not even sure how to describe this, so my research hasn't yielded much.
Take for instance the figure of speech "A million little things", which doesn't really mean literally one million things, but rather "a heck of a lot of little things".
What would you call "million" in this context? It has a meaning of its own, but it's used to just mean "a lot" rather than "a million". Bonus points I guess if the word can also be used to describe words without a meaning, like "gajillion".
Sample sentence: "In this context, 'million' is a ____, used to mean 'a lot'."
I may be muddying the waters here, but I think other examples would be words like "brother", which is sometimes used to refer to non-brother relationships, and "peanuts", which can be used to refer to non-peanut things that are perceived as insignificant.
With the latter two examples, "slang" seems appropriate, but calling "million" "slang" seems to be off the mark.