New to tubes-what the heck does NOS mean?

I checked 50 posts in the forum and saw many references to "NOS" but didn't see what it stands for. Tell me please?
Also, any opinions on good values in EL34 tubes? I've read some good things about EH (Electro-Harmonix) being a good output tube VALUE. Your comments based on actual listening experiences are appreciated.
Thanks and happy listening!