How well one can do a thing. Proficiency is built by practice and is specific to what was practised.
Latin proficientia (advancement, progress), from proficere (to make progress, to be useful): pro- (forward) + facere (to do, to make); proficiency was first a measure of advancing, only later a measure of skill held.
How well one can do a thing, built by practice rather than granted whole. The root's sense of advancing is apt: proficiency is progress made visible, not a fixed trait someone either has or lacks.