Leonardo da Vinci's wing-flapping flying-machine concept was an ornithopter.
An ornithopter is an aircraft designed to fly by flapping wings, rather than by using fixed wings or a rotating rotor. Leonardo studied birds and bats extensively and made numerous drawings of machines whose wings would be moved by human power. His notes examined wing shape, balance, muscles, and the movement of air.
Leonardo's ornithopter designs were imaginative engineering studies, not working aircraft. Human muscles cannot provide enough sustained power to lift a person through repeated flapping, and the mechanisms he proposed could not overcome that limitation. Even so, the drawings demonstrate his effort to connect observation of nature with mechanical design.
The term is broader than Leonardo's work. Modern ornithopters include small radio-controlled models and experimental aircraft, and some designs use motors instead of human power. An ornithopter is therefore not the same thing as a glider, which relies on fixed wings, or a helicopter, which uses powered rotary lift.