It can be one that you want to become a master in or maybe it is one that you wish you had time to even learn. Perhaps it is a language you've read about, but haven't even seen a line of code written in it.
Mine is C. I am proficient in C, but really wish I had the time to master it and truly know the in's and out's of.
After following along the with the whole "You should learn a new language every year" line of thinking for a few years, I realized most of the languages I was choosing were all fairly similar - Perl, C++, Java, Python, Ruby, Javascript etc. (Not to say I'm a master in all of them, but I'm at least somewhat proficient in most, and reasonably competent with Ruby and JS)
I'm currently working on trying to master a lisp variant just because it's different from what I know and use. I started off with CommonLisp using the "Land of Lisp" book, but found the lack of exercises in the text made it of limited value. I've now started with scheme using "Structure and Interpretation of Computer Programs" book - I'm finding this a lot harder due to the maths involved, but hopefully things should start to become clearer soon =)
I'm guessing after this, my target for next year will be something like Haskell.
I'm alright with low level stuff but I want to pick up Python so I can roll out prototypes after prototypes to run on Google App Engine.
That and Python is SO full featured. Libraries for everything from statistics to visualizing stuff to image manipulation!