In museum terms, an accession is an object (or group of objects) added to the permanent collection, all at once and from a single source. So, a gift (or purchase) of 20 paintings might be called an accession. Generally, if a donor gave a museum two groups of art, even in the same year, they would be considered two different accessions.
The term accession can also be a verb, indicating the act of processing an accession. This includes things like researching the work, writing a catalog entry for it, and assigning a unique identifying code called an accession number.
The opposite process of accessioning is deaccessioning. Museums do not "sell" artworks, they "deaccession" them.