What does PHI node mean? [GCC Internals Manual]

[Cheng Renquan]

Recently I was reading the GCC Internals Manual, found some
abbreviated words hard to understand, even google didn't
give any meaningful explanations;

the latest one is PHI, in chapters of "Passes, GENERIC, GIMPLE"

Before this PHI word, to find what SSA means also cost me much
time,

I think, should the gccint manual add a special terminology
index, and explains it at its first occurrence? or in the
comments of some source file is also ok, but currently it's
unlucky no explanations of PHI, no meaning;