by Kathleen Watson

According to merriam-webster.com, scientists in the mid-19th century needed a word to describe the most favorable point, degree or amount; the best condition for the growth and reproduction of an organism. They took “optimus” from Latin to create the noun optimum.

It filled the scientific need, and optimum eventually gained use beyond the scientific community to broadly imply the best or most desirable.

A few decades later, optimum was being used as an adjective as well as a noun. That’s when optimal was coined to serve as an adjective, but the distinction is either not understood or not accepted by everyone.

A popular resource for writers, Garner’s Modern American Usage, prefers “optimum” as the noun and “optimal” the adjective.

Noun Examples: optimum

These examples show how optimum is used as a noun (the best condition or amount):

Professor Albertson was pleased that the soil conditions of the test garden finally reached their optimum.

The pass interception yielded the optimum the coach could have hoped for.

Your thorough preparation resulted in the optimum your job search could have achieved.

Adjective examples: optimal

These examples show how optimal is used as an adjective (the most desirable, most favorable, most effective). Note that optimal is followed by the element it modifies:

Once students achieved optimal soil conditions for the test garden, the plants thrived.

The quarterback’s injury contributed to an optimal opportunity for a pass interception.

Because of your thorough preparation, your achieved optimal results from your job search.

I align with those who recognize and appreciate the distinction between optimum and optimal. Consider these pairs of words that follow the same noun / adjective pattern as do optimum and optimal:

bacterium / bacterial

cerebrum / cerebral

cranium / cranial

minimum / minimal

If optimum and optimal mean the same thing — if they are interchangeable — why do both words exist? Choosing one word over another because of its precise meaning or nuance separates the thorough writer, editor, or publication from the rest.

Kathleen calls herself the Ruthless Editor. She has created Grammar for People Who Hate Rules to help people write and speak with authority and confidence. Kathleen can be reached at kathy@ruthlesseditor.com.

In your opinion, are these two words interchangeable?