Comment on Nabataea by Nemo-the-Everbeing
I know, i's awesome! But I suppose Galifrey wasn't even named much less defined all that well until at least the third Doctor, Susan mentions it once and that's it!
Gallifrey and the Time Lords first got a bit of specification in "War Games", the last of the Second Doctor adventures. It's the first time we find out what the Doctor is, where he comes from and what Gallifreyan society is like.
I thought the planet wasn't actually named until the thrid Doctor though...oh well what do I know...I get all my Gallifrey know how from the internet. There's this great site called 'Rassilon, Omega and that other guy' which has just about anyhting you could possibly want to know about Gallifrey in it. There's even the laws of time, it's pretty funny scrolling through them going "Yep, he's broken that one, that one, that one half a dosen times"
I'm almost certain it was in the last Second Doctor adventure (they go to Gallifrey, so I can't imagine it wasn't actually mentioned by name). They might consider it Third, as it was very late in that adventure that this all went down, and it's very frequently mentioned in the Third Doctor's adventures, as he's in exile to earth from there.
Okay then second Doctor it is!