Decan - from the latin word, decius, means ten. First decan is the first ten degrees of any sign.
There are 12 signs and each sign is 30 degrees of an arc. Each sign has three decans: the first ten degrees of a sign are the first decan, the second ten degrees of a sign is the second decan, and the third decan are the last ten degrees.
I hope that makes sense, I'm kind of a looney with these lately. sweatdrop
Also, my apologies for the late reply, I haven't been on here as much as I used to... ^^;