Q: Is there a difference between “instantly” and “instantaneously”?

A: Yes and no, and whatever distinction exists between these two adverbs is getting less distinct.

Both words can mean in an instant, immediately, or at once, but “instantaneously” has another meaning: it can refer to two events that occur at the same or virtually the same time.

For instance, you can say “The corporal obeyed instantly” or “The staff sergeant obeyed instantaneously” (though “instantly” sounds more idiomatic to our ears).

But if you want to indicate that the two orders were carried out at the same time, you’d say, “They were obeyed instantaneously.”

A bit of googling, however, suggests that this distinction between “instantly” and “instantaneously” is being lost.

That may explain why R. W. Burchfield, author of Fowler’s Modern English Usage (rev. 3rd ed.), dropped the “instantly”/”instantaneously” item added by Sir Ernest Gowers in the second edition. Here’s what Gowers had to say:

“Instantly is virtually a synonym of at once, directly, and immediately, though perhaps the strongest of the four. Instantaneously is applied to something that takes an inappreciable time to occur, like the taking of an instantaneous photograph, especially to two events that occur so nearly simultaneously that the difference is imperceptible.”

Bryan A. Garner, writing in Garner’s Modern American Usage (3rd ed.), generally agrees with Gowers.

But the only standard dictionary we found with an entry for the adverb “instantaneously” doesn’t make the distinction. The Collins English Dictionary defines it this way:

(1) “in a way that occurs with almost no delay; immediately ⇒ Airbags inflate instantaneously on impact.” (2) “in a way which happens or is completed within a moment.”

Of the two adverbs, “instantly” is the oldest. When it entered English in the 15th century, according to the Oxford English Dictionary, it meant “urgently, persistently, with importunity,” but this sense is now considered archaic.

In the mid-16th century, it took on the modern meaning we’re discussing: “In a moment; immediately, forthwith, at once.”

“Instantaneously” showed up in the mid-17th century, according to the OED, with the meaning of “in an instant, in a moment; without any perceptible interval between beginning and completion.”

Interestingly, the OED entry for “instantaneously” doesn’t mention the distinction cited by Gowers or Garner, and it has no published reference for the usage.

Our Google searches suggest that many, if not most, English speakers are unaware of this distinction.

As we’ve said, we think “instantly” sounds more idiomatic than “instantaneously” in describing something that happens immediately.

And if we wanted to indicate that two events occurred at the same time, we’d say they occurred “at the same time” or “simultaneously.”

Check out our books about the English language