Backdoor

Taken from MalwareFox

7. Definition: A Backdoor (also called a trapdoor) is a malicious software which is installed on a system or application with the intention of the attacker to access the system at a later date via a hidden feature or command in the software.
   
   A backdoor may be installed through a Trojan. These are usually in stealth mode and get activated by the attackers based on their intentions.
   
   Reason for the name: Just as not everyone is aware of entrances into a house besides the front door, such as a backdoor, most people are also not aware of the hidden feature in the software.
   Propagation: When used in a normal way, the software performs completely as expected and advertised (e.g., a game). However, if the hidden feature is activated by the attacker (possibly remotely,) the software does something unexpected, such as getting admin privileges in the OS.
   Examples of well-known backdoors: the SolarWinds hack and the WordPress hack.