Instance in Appian

When we talk about processes, then an active process can be called an instance. Is this what you mean?

I would say that a process instance is the representation of an individual execution of a process model. It can have multiple statuses.

In general an Appian instance for me sounds more like a complete Appian platform. A customer for different reasons might have multiple Appian instances (for example one in the cloud and one on premise, or two different on premise Appian instances, which is not usual but can happen)