First introduced at Paris in 1878, adjournment is a time out between playing sessions, generally overnight. The side that is on the move seals (writes on a piece of paper and seals it in an envelope) a legal move, sight unseen by his opponent. The envelope containing the move is kept safe by the arbiter.
When play resumes, the sealed move is played on the board, and the game continues. Players were forbidden to analyze their games during adjournments, but this became difficult to enforce. In the 1930s analytical assistance by seconds for adjourned games became acceptable.
In the modern era, adjournments are rarely used because of the possibility of computer assistance during the adjournment.