Available Definitions:
1) n. - Fitness; suitableness; appropriateness; as, the aptness of things to their end.
2) n. - Disposition of the mind; propensity; as, the aptness of men to follow example.
3) n. - Quickness of apprehension; readiness in learning; docility; as, an aptness to learn is more observable in some children than in others.
4) n. - Proneness; tendency; as, the aptness of iron to rust.