A person who takes something without permission or legal right and without intending to return it. The stealer was caught on camera taking the bicycle from the front yard.
In sports, particularly baseball, a player who is skilled at stealing bases. The team's best stealer managed to swipe second base with ease.
A piece of software, device, or other agent designed to covertly acquire data, credentials, or other valuable information. The security team detected a password stealer lurking in the system.