In general, a mod is any device that has replaceable batteries inside of it. Some will define it a little more narrowly to include only those devices without electronic regulation.
A mod is kind of like a hot rod. A few years ago, when only only cigarette sized electronic cigarettes were available, vapers quickly discovered that more power and longer battery life would make vaping more enjoyable. Many of us "modded" flashlights, pipes, battery boxes and even cell phones to provide the power that we wanted. Since then, manufacturers have begun turning out similar devices in large quantities, but whether they are still "mods" is an open debate. Is a new Mustang fresh off the showroom floor a hot rod, or is that term reserved for a one-of-a-kind creation from someone's garage?