Which term is used for malicious software that can remain hidden and provide unauthorized access to the system?

The term "Backdoor" refers to a type of malicious software designed to provide unauthorized access to a system without the user's knowledge. Backdoors can be utilized by attackers to bypass standard authentication processes and gain remote control over the affected systems. This allows them to access and manipulate data, install additional malware, or exploit the system further without being detected.

In contrast, other options represent different categories of malicious software. A Trojan masquerades as legitimate software to trick users into installing it, but it may not inherently provide a hidden path for unauthorized access. Spyware is designed specifically to collect data from users' systems without their consent, such as tracking keystrokes or harvesting sensitive information but does not necessarily provide direct access to the system. Ransomware encrypts a user’s files to demand a ransom for access but does not serve as a means for ongoing unauthorized access after the initial attack.

Thus, "Backdoor" encompasses the characteristic of remaining hidden while granting unauthorized access, making it the most appropriate term in this context.