Mathematicians have wrestled with this question (which is more complex than most laypeople would likely think), but now it turns out that lexicographers have, too. "The cardinal number equal to the sum of 1 + 1," dictionary.com gives as the first definition.

Maybe I shouldn't find the definition odd, but I do -- it just seems so utterly pointless in a dictionary, even if completely accurate. (A non-English-speaker might need to translate the word into the number, but "2" would have sufficed for that.) Yet of course they needed something accurate, even if it didn't really help the readers much.