Full Idea
A part of duration wherein we perceive no succession, is that which we may call an 'instant'; and is that which takes up the time of only one idea in our minds.
Gist of Idea
An 'instant' is where we perceive no succession, and is the time of a single idea
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.14.10)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.185
A Reaction
Given that the present appears to have zero duration (if it is where past and future meet), then this strikes me as a pretty accurate account of what we mean by an instant.
Related Idea
Idea 1902 Since Socrates either died when he was alive (a contradiction) or died when he was dead (meaningless), he didn't die [Sext.Empiricus]