Is the heart an organ or a muscle?

Both: Organ: a part of an organism that is typically self-contained and has a specific vital function, such as the heart or liver in humans. Heart: a contractile organ consisting of a special bundle of muscle tissue, which moves blood though the arterial system of the body.

Both, sort of: An organ is a collection of tissues joined in a structural unit to serve a common function. The heart is made up of cardiac muscle, connective tissue, blood vessels, and other tissues that work together to pump blood throughout the body.