Q: Which is the correct verb to refer to cohabitation? Is it cohabitate or cohabit? Are they interchangeable? Or do they differ in meaning? If so, what is the difference?

A: These questions, submitted by a Chicago lawyer, demonstrate one habit of speakers of English, the tendency to create “backformations.” This is how backformations come about: first a verb enters the English language, sometimes borrowed, sometimes coined, sometimes derived. In the case of the verb cohabit, the verb was derived from the late Latin infinitive cohabitare that combined the prefix co “together” and habitare, “to dwell.” The English verb means “to live together as husband and wife without being married.”

Then, as usually occurs, English speakers created the noun cohabitation from the verb to cohabit. The noun was probably hyphenated at first, then it become a single word. Then came the backformation: From the noun that had been created from the verb, a new verb was created from the new noun, giving us the needless, redundant, and unwieldy concoction, to cohabitate.

Backformation is common. It has provided many unnecessary verbs. For example, the verb orientate was formed from the noun orientation that had been created from the original verb to orient. That verb orient had an interesting background, entering Middle English from Old French, which got it from Latin. The noun orient became a verb, meaning “to adjust or align toward the east,” which then broadened to mean to adjust in any direction or situation. Typically, the verb orient added -ation when it then became a new noun, orientation. And, again typically, the original verb was ignored and a new verb, to orientate was created from the noun.

Backformation also created the verb to administrate. The process started with the verb administer, from which the noun administration was created. Then came the new verb administrate, longer, redundant, and unneeded. What happens then is that, typically, unless the two words differentiate in meaning, one disappears. And, unfortunately, the earlier and shorter verb tends to disappear. That is now happening with the verb to illumine. After the noun illumination was created, backformation gave us the new verb to illuminate. How often do you now hear the original verb to illumine? Our verbose society apparently prefers long words.

But new verbs are often created because they are needed. For example, the verb to recidivate came from the noun recidivism. A reader wrote to criticize recidivate after seeing it in a legal journal in the context, “Only 2% of probationers recidivate.” But it seems useful for expressing the noun recidivism as a verb.

Another reader complained about the verb to aggress, which he saw in The Chicago Tribune sentence, “Haiti ... has aggressed against none of its neighbors.” Both recidivate and aggress are listed as standard in current on-line dictionaries (2005 and 2006), though the 1985 American Heritage Dictionary does not list recidivate and says of aggressed, that it “has lately become associated with the jargon of psychology and is often objected to.”

Some new backformed verbs are accepted with alacrity, one being to morph, which has been approved by the American Dialect Society (an association of academics and other English language afficionados). To morph is a backformation of the noun metamorphosis (“a transformation, often by sorcery”). Popular acceptance has also been widespread, perhaps because the new verb does not replace a previous verb, and provides a needed one-syllable replacement for the longer phrase, “having been transformed.”