In electrical terms, what is the definition of a "disconnect"?
National Electrical Code 2014
Chapter 1 General
Article 100 Definitions
Disconnecting Means. A device, or group of devices, or
other means by which the conductors of a circuit can be
disconnected from their source of supply.

This can include a switch, a circuit breaker, a cord and plug connection (sometimes), or a group of the aforementioned devices.
There are other stipulations within the code that dictate how these devices can, and cannot be used. But at the core of it, any device that can disconnect the conductors of a circuit from their source of supply, is a disconnecting means.
In the context of a separate building, typically either circuit breakers or a switch type device will be used.