That certainly depends on what you want to mean. Natural rainfall has a lower pH limit of about 5.5 (because of CO2 in the air making carbonic acid; just a matter of how close to equilibrium the water has gotten whether the pH is actually as low as the mid 5s), so "unusually" acidic precipitation would be stuff that is markedly below that. Normally, when someone talks about acid rain, they actually mean unusually acidic rain, so rain with a pH significantly below about 5.6 (plus or minus a few tenths of a pH unit).
If I came across that question on a test or quiz in school, I would simply spit back whatever I had been told. If I could not remember what I had been told, then I would go with "below 5.6".
Strictly speaking, any rain at normal temperature and pressure conditions (earth surface conditions) is "acid" if the pH is lower than 7.