Assay: An assay is an analysis done to determine:

1. The presence of a substance and the amount of that substance. Thus, an assay may be done for example to determine the level of thyroid hormones in the blood of a person suspected of being hypothyroid (or hyperthyroid).
2. The biological or pharmacological potency of a drug. For example, an assay may be done of a vaccine to determine its potency.

Assay is also used as a verb and has several related meanings in this role:

1. To try or attempt. For example, "she assayed this operation for the first time and so was understandably nervous."
2. To analyze a mixture for one or more of its components
3. To judge the value, the worth of something.

The words "assay" and "essay" are often confounded, a cardinal sin according to some word mavens. However, "assay" and "essay" share a common ancestry. Both words come from the French "essai" which in turn came from the Late Latin "exagium" meaning the "act of weighing".