Who says there's a distinction?The way I see it, if I'm super technical about it... "disbelief" implies knowledge of the concept's existence. "Lack of belief" implies... lack of knowledge. So like... a baby lacks belief in God. I disbelieve in God.Or maybe "disbelief" implies a level of reluctance to believe in something. Like, "I averted my eyes in utter disbelief." But I think the same connotation can be granted to "lack of belief" as well.
Maybe disbelief implies an active disbelief (like that of a militant atheist) and "lack of" implies passivity.
Apparently there is a distinction between the two, but I honestly don't see it. A disbelief in God is exactly identical to a lack of belief in God, and doesn't necessitate a belief in God's non-existence. At least, that's how I see it.