I get asked this a few times so here is how our team view these definitions.
A mobile device is defined from a user context point of view as a device that can be comfortably used, for normal use whilst on the move or standing up. Note that a tablet device and any wearable device mobile devices. A small notebook such as the Asus Eee PC 700 measuring in at 28cm in the diagonal (and actually physically smaller than the Nexus 10 tablet) is not a mobile device since you would typically place the device on a table or on your lap to use it. PCs with detachable or flippable screens that such as Asus Transformer and VAIO Flip are not mobile devices when the physical keyboard in place.
A tablet device is a large mobile device that does not comfortably fit in one hand. From a user context point of view, a user is less likely to quickly pull a tablet out of their pocket in a crowded area, whereas they would for a non-tablet mobile device. To be precise, a tablet is a mobile device that is physically larger than 18cm measured diagonally