I had an idea for a game, I'm not sure how well it will work though. The idea is to post a Japanese definition of a word, and other people try and guess it. The person who gets it right posts the next definition.

To keep it easier for people who aren't really advanced, I would say limit it to something on this list (about ~1500 vocabulary):
http://www.tanos.co.uk/jlpt/jlpt4/vocab/
We could expand it later if it ends up being too easy.

You can get definitions here:
http://dictionary.goo.ne.jp/
http://dic.yahoo.co.jp/

Here is the first definition for someone to guess:
周囲が水で囲まれている陸地。

島？

Correct.

駆動用電動機を装置し、架線あるいは軌道から得る電気を動力源として走行する鉄道車両。

電車？

新幹線？

it was 電車

宗教を同じくし、その教えを守り伝えようとする人々の団体。また、その教えを述べ、礼拝儀式を行うための建物。

Oops, RTFM, also no delete post anymore?

Well, it's a church. That I know. Is it 教会?

Guessing the exact word might get tricky. Nevertheless, OP you're a genius.

Correct!