An accession number is a unique code used to identify items in a collection. Accession numbers are used in many fields, but in museums, they identify artworks and other objects in the permanent collection.
Accession numbering schemes differ from one museum to another, but they usually consist of several parts, which indicate what year the item was acquired, which gift it was part of, and what order it was processed in that gift. An accession number might look like this:1972.11.23
This would indicate an item that was given to a museum in 1972, and was the 23rd item to be processed in the 11th acquisition of that year.
Accession numbers are often listed on the identification cards in museums, which allows you to see when they acquired each object.