Q: I see this kind of sentence all the time in The New York Times, The New Yorker, etc.: “A solar eclipse is when the moon blocks the sun.” My English teacher in high school would get very angry at the use of “is when” in definitions. Is it acceptable now? Shouldn’t the “is” be followed by a noun?
A: Although it was once common to use “is when” in definitions (for example, “Poverty is when your stomach is empty”), the usage has been considered colloquial since the mid-19th century. It’s common in speech, but good writers generally avoid it.
Usage authorities argue that a noun or gerund, not an adverb like “when,” should follow “is” in a definition. So it would be better to write “Poverty is an empty stomach” or “Poverty is having an empty stomach.”
Nevertheless, people have been using “is when” (and “is where”) in definitions for many centuries. I did a search of quotations in the Oxford English Dictionary for “is when” definitions and came up with hundreds, going back to the 14th century. Here’s a sampling.
1340, in The Psalter of Richard Rolle of Hampole: “The inspirynge of his ire is when he says stilly in oure hert ….”
1547, in a medical book by Andrew Boorde: “Abhorsion is when a woman is delyvered of her chylde before her tyme.”
1719, in John Quincy’s Lexicon Physico-Medicum: Or, a New Physical Dictionary: “Alcalization is when any Liquor is impregnated with an alkaline Salt.”
1788, in Thomas Reid’s A Brief Account of Aristotle’s Logic: “Begging the question is when the thing to be proved is assumed in the premises.”
So who first said the usage was grammatically incorrect?
The earliest objection I could find was in an 1851 handbook, The Grammar of English Grammars. The author, Goold Brown, says neither “when” nor “where” is “fit to follow the verb is in a definition … because it expresses identity, not of being, but of time or place ….” Goold later cites dozens upon dozens of such misuses, most of them in books by other grammarians!
(I could have saved myself lots of research by looking in Merriam-Webster’s Dictionary of English Usage, which notes that Goold Brown was the first to condemn this usage.)
At any rate, these “is when” definitions are best avoided, in my opinion, especially in writing. They may be handy, but the definitions are not grammatically parallel to the things being defined.