An accessory is a piece of equipment that provides cosmetic changes to a character. Accessories may also increase a character's stats. They have been present since the very first Hyperdimension Neptunia and in every other installment past it.
Since accessories are primarily used for aesthetic reasons, they do not provide any worthwhile additional stats to a character when equipped. In Hyperdimension Neptunia and Hyperdimension Neptunia mk2, some accessories came with an additional stat increase in some area.
However, by Hyperdimension Neptunia Victory nearly no accessories provide any statistical increase to the character outside of special accessories such as DLC ones. This trend would carry on into later titles of the series.