Aptitude, n.
Etymology: [F. aptitude, LL. aptitudo, fr. L. aptus. See Apt, and cf. Attitude.]
1. A natural or acquired disposition or capacity for a particular purpose, or tendency to a particular action or effect; as, oil has an aptitude to burn. He seems to have had a peculiar aptitude for the management of irregular troops. Macaulay.
2. A general fitness or suitableness; adaptation. That sociable and helpful aptitude which God implanted between man and woman. Milton.
3. Readiness in learning; docility; aptness. He was a boy of remarkable aptitude. Macaulay.
Shades of meaning
Fernald has no entry of its own for aptitude; he lists it under dexterity, power.
Where he writes about it:
Aptitude (L. aptus, fit, fitted) is a natural readiness, which by practise may be developed into dexterity. — from dexterity