Backdoor

A backdoor is a deliberately hidden piece of code that gives an attacker access to the site while bypassing normal login. It is left behind after the first breach, precisely so the attacker can come back the moment the owner "tidies up".

This is why simply removing the visible infection is usually not enough: if even one backdoor remains in the files, the attack returns within days. Effective cleanup means finding and closing every backdoor as well as the flaw the attacker used to get in.