A "leap second" is a single second of time which is sometimes added into the Coordinated Universal Time in order to make it more accurately sync up with astronomical time. Leap seconds are added on an as-needed basis, and are typically tacked on as the 61st second of the last minute of June or December. The time of these additional seconds are notated as 23:59:60.

Related Fact: The most recent leap second was added on December 31, 2008. So, if that felt like a long year, now you know why.

[Source: Time and Date. See previous Numbers of the Day here.]