A piece of hardware or software that captures a user's keystrokes is known as a:

The term used to describe a piece of hardware or software that captures a user's keystrokes is indeed a keystroke logger. This specialized tool is designed to monitor and record every key that a user types on their keyboard, which can include sensitive information such as passwords and personal messages. Keystroke loggers can be implemented as standalone hardware devices or as software programs hidden in the operating system, making them particularly challenging to detect.

Understanding the context of this malicious activity is crucial for recognizing the impact of such devices on cybersecurity and personal privacy. Users should be aware of their existence and the risks they pose, often leading to security breaches and identity theft.

The other options do not accurately describe keystroke loggers. An informant typically refers to a person who provides privileged information about something, rather than a tool for recording keystrokes. A virus is a type of malicious software that replicates itself and spreads to other computers, often causing damage but not specifically focused on capturing keystrokes. A Trojan, on the other hand, disguises itself as legitimate software to trick users into installing it, but it may include various malicious capabilities that do not solely revolve around keystroke logging.