Thanks, PDR, for the helpful links.
If I am understanding correctly, airframe icing can't occur in clear air as water vapour needs to be supercooled - that is, in liquid form below zero degrees celsius. Therefore, for the sake of general aviation, and when considering flight under IFR (with an IR(R) or similar rating), icing could be considered as flight in cloud (or precipitation) below zero.
Is that a fair (and safe) summary?