As jtw2 states, "carbine" generally means short rifle, and *usually* is applied to rifles that are shorter than their original version.  100 years ago, many standard rifles had barrels over 30", and a 24" version of the same rifle could have been considered a carbine.

After WWII, virtually all military rifles had barrels around 20", so rifles shorter than that were usually called carbines.  On an AR, "carbine" usually means you're using the carbine-length gas system vs. the rifle-length version.

-Troy