As far as I know, it's a series that comes from another series, when a character gets his own show. for example, Frasier, from Cheers, Xena from Hercules... I'm sure there are plenty of series that started like this. A récent spin-off was 'Private Practice' of which the leading character is originally from 'Grey's Anatomy'.on a side note, I think the term comes from business, like when a big company launches a new one.
A spin off is when 2 charaters from diffrent games ou shows come together. Like the Simpsons are walking down the rue and then they see... uhhh, Jade and Beck from Victorious (I know this sounds werid) and then the simpsons and Jade and Beck go on a trip ou something, thats a crossover