What is the analogue of "molecule" for ionic compounds?
The term you are looking for is formula unit, I think. Wikipedia doesn't really describe it super well, but just to give an example, you could write the sentence


  $\pu{58.44 g}$ of $\ce{NaCl}$ contains $6.022 \times 10^{23}$ formula units of sodium chloride


and it would be pretty well understood.

See also: What is the difference between a Chemical Formula and Formula Unit?