Which definition best describes backdoor malware?

A backdoor is a type of malware designed to bypass normal authentication procedures to gain unauthorized access to a computer system or network. It creates a hidden entry point, allowing an attacker to remotely control the compromised system. The other options describe different types of malware: a "malicious program used to launch other malicious programs" could refer to a dropper or a loader; "malicious code that infects a user machine and then uses that machine to send spam" describes a bot or a spam bot; and "malicious code with the main purpose of downloading other malicious code" is characteristic of a downloader.