In 'Elements of Statistical Learning' Chapter 14 (p. 503), objects $i$ and $i'$ and further $x_i$, $x_i'$ are commonly referred to. However I don't think they are explicitly defined. What could be the meaning of this symbol: $'$
I think it could possibly be that $x_i$ and $x_i'$ are different objects in the same set but am not sure.
Source: Friedman, J., Hastie, T., & Tibshirani, R. (2001). The elements of statistical learning (Vol. 1, No. 10). New York: Springer series in statistics.