The definitions for the term "potency"

The potency is a measure of drug activity expressed in terms of the amount required to produce an effect to given intensity. The more potent a drug the lower the amount required to produce the effect.

The term relative potency is used to express the biological activity of a sample preparation compared to a standard preparation.

In chapter 5.3 of the European Pharmacopoeia 2005 the following definitions for the term potency are given:

"stated potency" or "labelled potency"
In the case of a formulated product a nominal value assigned from knowledge of the potency of the bulk material; in the case of bulk material the potency estimated by the manufacturer.
"assigned potency"
The potency of the standard preparation
"assumed potency"
The provisionally assigned potency of a preparation to be examined which forms the basis of calculating the doses that would be equipotent with the doses to be used of the standard preparation.
"potency ratio"
The ratio of equipotent doses of the standard preparation and the unknown preparation under the conditions of the assay.
"estimated potency"
The potency calculated from assay data