1A pendulum consisting only of a bob suspended by a cord or wire, specifically: (a) (in a clock) one which is not equipped with a mechanism that compensates for the effects of heat, friction, etc.
2A theoretical or ideal pendulum, consisting of a single point that has mass but no extent, is suspended by a massless inextensible rod, and moves without friction.
simple pendulum/ˌsɪmpl ˈpɛndjʊləm/ /ˌsɪmpl ˈpɛndʒʊləm/
Late 17th century; earliest use found in Philosophical Transactions of the Royal Society of London. From simple + pendulum.