What does apostrophe-like symbol in the superscript mean? It's hard to look up the meaning of a symbol if you don't know what it is called in the context that it is written.

That apostrophe-type symbol in the sequence notation - what is that?
That's a prime.


  In mathematics, statistics, and science, the prime is generally used to generate more variable names for things which are similar, without resorting to subscripts – $x'$ generally means something related to or derived from $x$.