The phrase 'In a Split Second' refers to an instant.
Example of Use: "For a split second, it looked like she would fall."
Interesting fact about In a Split Second
A second was the smallest measure of time until the 1884 origin of the idiom 'split second'. It was then that a stopwatch with two independent second hands that could be stopped at different times was invented; and at the same time, the idiom 'split second' came into use. A related idiom, 'fraction of a second' was popularized in 1912.
