1
general::
An instant, a fraction of a second, as in Our best swimmer came in a split second before theirs. This expression alludes to a stop watch that has two second hands, one above the other, for timing more than one athlete or intervals of a race by a single athlete. Each hand can be stopped independently of the other, so a second can be "split" when one second hand stops a fraction of a second after the other. [c. 1880]
American Heritage Idioms
2
general::
Phrase(s): *a split second
an instant; a tiny period of time. (*Typically: for ~; in ~.) • The lightning struck, and in a split second the house burst into flames. • For a split second, it looked like she would fall.
McGrawhill's American Idioms And Phrasal Verbs