I am currently reading a paper from 2021 which uses "perfection" of schemes over finite fields. If $X$ is such a scheme over $\mathbb F_q$, the associated perfection is denoted by $X^{\mathrm{perf}}$. The whole process is used without reference, so that the construction seems to be well-known in that area of algebraic geometry. However it is the first time that I encounter this.
Would somebody understand what kind of space $X^{\mathrm{perf}}$ is, and could recommend some material to learn more on the perfection process?
Just to get this off of the unaswered list, a good place to start reading about this topic is Appendix A of the following paper.
Zhu, X., 2017. Affine Grassmannians and the geometric Satake in mixed characteristic. Annals of Mathematics, 185(2), pp.403-492.