This really depends on the situation/context (and there is no *one fits all* solution). 

In general i,j,k [btw, **i** can be taken as **index**; its usage comes from mathematical background where those were most often the first three indexes (remember tensor calculus). It may also be connected to Fortran's history, where **i** being the first implicitly typed integer, was often used as an index of a loop. ] are often used if it is a simple counter. If it has a specific meaning, like **month** or **year** then longer variable name is better.