Android boss Andy Rubin offered up a pretty impressive number yesterday, relaying on Twitter that some 3.7 million Android devices had been activated on Christmas Eve and Christmas day, roughly double the platform's typical activation rate. But the question is, what constitutes an "Android device," exactly? On some level, the Kindle Fire, Nook Color, and Nook Tablet are all "Android devices," for instance — but considering how far removed they are from Google's first-party ecosystem, it seems unlikely that Rubin would be including them. And even if he was, how would he have access to that information? All of those devices are built off Android Open Source Project code with seemingly no link to a Google server that would be able to register the existence of new devices in the field.

The question was bound to come up. Rubin offered some clarification on Google+, saying that "we count each device only once (ie, we don't count re-sold devices), and 'activations' means you go into a store, buy a device, put it on the network by subscribing to a wireless service." Of course, that would imply that an "activation" only applies to a device that functions on a cellular network — and seeing how many Android tablets only have Wi-Fi, that doesn't seem like a solid metric. Certainly, Rubin (and Google) wouldn't want to sell the number short.

We've now gotten some additional clarification from trusted sources on what Google considers an "Android device" for the purposes of counting activations (which would presumably apply to every activation count Google has released in the past). It's actually really simple: you need to activate Google services on the device. In all likelihood, Google's counter actually jumps the moment you sign into your Google account on the phone or tablet, whether that be the first time you turn it on or when you're prompted after jumping into something like Gmail or the Android Market. And as Rubin says on Google+, it only happens once per physical device.

To look at it a little differently, that means that the Kindle Fire, Nook Color, Nook Tablet, and other devices built off AOSP code without access to the typical "Google experience" apps definitely are not being included — but virtually every phone and tablet sold by a carrier in the US and Europe should be.