App Monitoring
Web Shell
Also: webshell
A small script an attacker plants on a web server to run commands through ordinary HTTP requests.
In more detail
After uploading or injecting a file such as a PHP page that executes whatever it is sent, the attacker keeps control without any open port. Requests to unfamiliar script files that return command output are the usual sign. Finding one means the server is already compromised: remove it, find how it got there and rotate every secret the server held.