Word Choice – Instantly Versus Instantaneously

Mary’s question: “My current pet peeve is the use of the word instantaneously rather than instantly. It seems rather cumbersome and the words, to me, are synonymous.”

BizWritingTip response:  Both adverbs are derived from the noun instant (Medieval Latin word instantāneus). And although they tend to overlap, the nuances are different.

Instantly means immediately. It implies promptness.
We phoned for the police, and they arrived instantly.
She instantly began to work on the report.

Instantaneously means occurring or operating in an instance. It implies speed as well as immediacy. Although I have seen it in standard business writing, it is more commonly used in technical writing.

Combustion occurred instantaneously.

Note: In some situations, either word could work.

He got up instantly. (right away)
He got up instantaneously. (all of a sudden)

Years ago, a teacher told me about the two-second rule. If the effect occurred within two seconds, you can say it happened instantaneously. If the effect happened quickly but after two seconds, it happened instantly.

Here’s another way to look at it: I answered the email instantly. However, I did not reply to it instantaneously as I spent a few minutes organizing my thoughts.