An elective monarchy is a monarchy whose reigning king or queen is elected in some form.
Also in Polish, after the death of the last Piast in 1370, Polish Kings were initially elected by small council. Gradually, this privilaged was granted to the all members of the gentry. Kings of Poland during the times of Polish-Lithuanian Commonwealth 1569-1795 were elected by gathering of crowdes of nobles at the field in neighbourhood of Warsaw. Every out of est. 500,000 nobles could have participated in the election in person.
At the start of the 20th Century, several monarchs of newly-independent nations were elected by parliaments. Without a well-established hereditary royal family, new nations often chose their own monarchs from among the foreign or domestic nobility in hopes that a stable hereditary monarchy would eventually emerge from the process. The now-defunct royal families of Sweden and Germany were originally appointed in this manner.
Other monarchs, such as the Shah of Iran have been required to undergo a parliamentary vote of approval before being allowed to ascend to the throne.
Currently, the world's only truly "elective monarchies" are in Malaysia, where the King is elected to a 5-year term by and from a small group of local hereditary rulers, and in Vatican City, where the Pope is elected to a life term by and (and usually from) the College of Cardinals.