Amazing what does 'rub off' isn't it - I know more about the inside of a car engine than I ever thought I would need ! Mind you, now they're computerised it's not much help !
Now car engines are a totally closed book to me....and that is how they are going to stay.
As to the original question, I think it is referring to divalent nitrogen-nitrogen bonds....bet that has clarifed it....
http://www.wordnik.com/words/azo-
Blimey ( would be more emphatic but don't want the filter changing it to broccoli or something)
Ladyalex brilliant answer! Has taught me something new today.