A ghost subscriber is someone who initially subscribed to a newsletter but doesn’t read messages anymore by systematically deleting them or by having their reception blocked using the “block sender” or “spam” commands of his webmail or email service.
By not using the unsubscribing link and not generating any error messages (except the feedback loop or a specific bounce), this subscriber usually stays in the subscriber’s list on the same account as still active subscribers. This is why we call this a ghost subscriber.
By not cleaning their lists, managers of particularly old newsletters may have a high proportion of ghost subscribers. It mechanically provokes a gradual decline of the observed open rate.