Instantly means at once or immediately. Instantaneously is variously assigned several meanings, some of which conflict with each other, but several references sources agree on a primary definition: happening or exerted with no delay in relation to something else. For example, one can tweet instantaneously about live sporting events, and a high-tech car can react instantaneously to changes in road conditions. This sense is indisputably the one usually meant in science, where, for example, Isaac Newton called gravity instantaneous because he believed gravitational force is exerted everywhere in its system in the same instant (this was later found to be false—whereas Newton’s gravity has an infinite speed, the influence of gravity actually moves at the speed of light).
Outside science, though, instantaneously takes other meanings. Some reference sources say it describes things that happen instantly and are over quickly, others say it describes multiple things that happen quickly and at the same time, and others define it in almost exactly the same way as instantly. So while drawing distinctions between them would certainly be useful, there is no consensus on the distinctions outside science, and in general usage the words are very often used interchangeably.
The examples below are all recent, and they’re drawn from a few different types of nonscientific texts. In our view, these examples support the idea that the two words are often used interchangeably, though no doubt other interpretations are possible (and of course it would be possible to pick out examples to support virtually any view on the words’ differences):
I don’t use Twitter often, but I like how my Tweets appear instantly on my Facebook page. Buzz delays them for hours. [Boston Globe]
[M]any of these children have grown accustomed to having adults (and often peers as well) instantaneously dismiss their concerns. [The Explosive Child, Ross W. Greene]
When an engine is running on an external tank, it stops when there’s no more fuel. The pilot must instantly switch to an internal tank, then lower the fighter’s nose to hold airspeed. [Aviation History 1]
That’s a desperately hard trick to pull off when one fifth of humanity, having previously subsisted on 7 percent of the world’s freshwater supply, decides that it wants to instantaneously increase its caloric intake. [Esquire 2]
With the sale of Brek Shea, Cooper instantly becomes the local face of the franchise. [Major League Soccer]
The problem is that the raunchy trailers pop up on sites without age restrictions almost instantaneously. [NY Times]
In scientific texts, instantaneously describes things that occur with no perceptible delay in relation to something else:
Electrons have zero mass and respond instantaneously to the applied potential. [Surface Modification and Mechanisms]
This indicates that though voltage changes instantly, current through inductor cannot change instantaneously. [Electronic Circuits]
With synchronous communications, the receiver gets the message instantaneously, when it is sent. [Principles of Information Systems]
1. Lyons, William. “P-51 Pilot: A Day in the Life.” Aviation History 23, no. 4 (March 2013): 36. ↩
2. Barnett, Thomas P. M. 2011. “When China Ruled the World.” Esquire 155, no. 1: 50. (March 2013): 36. ↩