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What is the difference between instant and instantaneous? Which is correct in this sentence?

It had an almost __ response time.

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They are nearly identical. Instantaneous focuses more on something occurring in an instant or instantly, whereas instant simply means "immediately".

However, being that both words necessarily involve time, your sentence would be redundant. Might I suggest one of the following:

It had a near zero response time.

It responded (almost) instantly/instantaneously.

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Based on the fact that manufacturers of instant coffee never call it 'instantaneous coffee' I would say that 'instant' modifies a noun referring to a thing, whereas 'instantaneous' modifies a noun referring to an event. Your example falls in a grey area between the two, because the noun 'time' can refer to either the duration before the response occurs or the event of the response's occurrence.

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