What Are Inactive Subscribers?
The most common explanation of an inactive subscriber is one who no longer reads your message, however, has not yet taken steps to halt receiving your email communications.
For example, say you send a few emails a week to list of 100 subscribers and only 50 of them open your emails as well as click on your links. The other 50 never open your emails thus never clicking on your links and never unsubscribing from your list. The latter half would be considered inactive subscribers.