Graham Morrison wrote :- " the initial for statement requires a variable which for some reason is nearly always called i in examples"
He is writing a bit tongue in cheek, but of course it stands for "integer".
More specifically the practice comes from Fortran, which was my first programming language. In IBM mainframe Fortran at least, all variables starting with letters between i and n were integers (geddit?) by default, and the rest were floating point. So many coders adopted this convention as a lifetime habit, and i is my own first choice for any counter.