A paradiddle is a four-articulation pattern that alternates hands for the first three articulations but then repeats the third hand. The two patterns are
L R L L and
R L R R. Fancier terminology would be "alternating single strokes followed by a double stroke."
I don't know where the name comes from, but I've just always assumed it was "drummer speak" and the "diddle" mimicked the repeated hand. This hypothesis is supported by the "paradiddle diddle," which would be
L R L L R R or
R L R R L L.
Edit: Be sure to check out Cort Ammon's very helpful comment below regarding the meaning of "para-": "Backwards engineering the word, "para-" is a prefix that can be used for "side by side," so perhaps that might have something to do with the origin of the word?"