I believe the term "MOD" is used to describe a PV that you can replace the battery in. However, there are certain devices that are probably considered mods that have non-replaceable batteries. Like the SD keyring, for example. Devices like attys, tanks, cartos, the Vivi Nova, etc... are devices that attach to mods and are not mods themselves. Mods refers to the device that powers such things.
ETA: If you were to ask this question, say, two or three years ago, the answer to "what is a mod" would probably be different. Then, the answer would probably refer to a DIY modification such as the Pyramid Tea Bag (PTB), or blue foam. Ecigs and their accessories of yesterday were very buggy, and often needed a bit of modification to perform better.