Potency (noun)

1. The ability to do something; capacity.
2. The degree of effectiveness of a particular drug or medicine; strength.

Origin:

Late middle english: from old french potencie, from latin potentia, from potent- "being able", from the verb posse "be able".

Examples:

1. The potency of the drug was tested in a series of clinical trials.
2. The drug's potency was found to be much stronger than the researchers had anticipated.
3. His speech had a potency that moved the audience to tears.
4. The potency of the poison was such that it killed the animal instantly.
5. The potency of the spell was such that it could only be broken by a powerful wizard.