'room'; Yiddish pronunciation: khéyder) is a traditional primary school teaching the basics of Judaism and the Hebrew language.
Where money was scarce and the community could not afford to maintain many teachers, boys of all ages would be taught in a single group.
After many Jews had fled to eastern Europe to escape medieval pogroms connected with the Crusades of that time, the intellectual centre of European Judaism moved with them and remained there for centuries.
This and the introduction of compulsory education eventually led to the dissolution of the cheder system, at least in Germanophone countries, although it continued to exist in Eastern Europe until as recently as the Holocaust.
In more Modern Orthodox Jewish communities in the Diaspora, chadarim (plural of cheder) are sometimes attended outside normal school hours.
Conservative and Reform Jewish communities, which are generally secular and assimilated, might have a similar program, but are more relaxed with regards to what they teach.