Apt"ness, n. 1. Fitness; suitableness; appropriateness; as, the aptness of things to their end.

The aptness of his quotations.
J. R. Green.

2. Disposition of the mind; propensity; as, the aptness of men to follow example.

3. Quickness of apprehension; readiness in learning; docility; as, an aptness to learn is more observable in some children than in others.

4. Proneness; tendency; as, the aptness of iron to rust.