"A fraction of a second would make them invisible?"
"A millisecond, a year, it wouldn't make any difference. If what we're reading is true, then we're occupying the same space but in a different time."
A millisecond was a unit of time equal to one thousandth of a second.
According to Spock, when V'ger attempted to communicate with the USS Enterprise, "its transmission was sent at a frequency of more than one million megahertz, and at such high rate of speed their entire message lasts only a millisecond." (Star Trek: The Motion Picture)
While USS Enterprise-D was in orbit of the last Tkon Empire outpost, Geordi La Forge observed that there was "a .372 millisecond delay between use of our power and the neutralizing counter force of the Ferengi." Using this information, along with the simple fact that the Enterprise's acceleration delay between slow-reverse impulse and top warp speed was .300 milliseconds, it was thought that they could escape the mysterious grasp that was on their ship by using that extra .072 milliseconds "to shift down and then kick hard into warp nine." (TNG: "The Last Outpost")