A Strength

1. The quality or degree of being strong.

Antonym: weakness

The intensity of a force or power; potency.

2. The strongest part of something; that on which confidence or reliance is based.
3. A positive attribute.

Antonym: weakness

4. (obsolete) An armed force, a body of troops.
5. (obsolete) A strong place; a stronghold.

ability

capability

expertise

fortitude

main

potency

power