Unlike past words-of-the-day that I've posted (read: vulpine - of or relating to a fox), today's WOTD is actually reasonable. It is...

abstruse: (adj.) difficult to comprehend.

Example sentence ~ "For me, the visible spectrum and diffraction of light are a piece of cake, but I find special relativity unbelievably abstruse!"

There's the word of the day - really use it today! :D