It isn't Andrea? How does that work, as IND allow it to be used it as part of your residency period...
Technically, any time you're in the UK, you have a 'visa' (as in, permission to enter/stay in the country), even as a visitor- but for Americans who visit the UK, this visa is just put in your passport at the airport rather than you applying for one in advance. So BUNACers in the UK, like all non-visa national visitors, are technically in possession of a UK visa granted at the port of entry, but the BUNAC card itself is not that visa, because visas are only issued by the UK government.
(at least, this is how I understand things from my research- anyone feel free to jump in and correct me if I've got it wrong!)
I don't think that the UK government wants details of every visit to the UK you've made when it asks the "have you been granted a visa" section since the visa at the airport thing is not how the term is usually understood. In terms of traditional entry clearance though, BUNAC doesn't really fall under that category. But like I said, it can't possibly hurt to disclose it anyway.