A flash cookie is a message used in Adobe Flash that is sent from a Web server to a Web browser and is then stored as a data file in the browser. Flash cookies behave like convential cookies by personalizing the user's experience, but they can hold much more data than conventional cookies. Flash cookies behave differently from conventional cookies in that they may stay installed on a drive after basic cleanup operations. They are also represented on a drive differently than the average cookie, so that many users don’t know they exist.
Websites may use flash cookies to improve the user's experience, but these cookies do pose privacy concerns because they can be used to collect information about how people browse certain websites.
Flash cookies may also be referred to as local shared objects (LSO).
Techopedia explains Flash Cookie
The use of flash cookies by various websites has created some controversy because of their ability to gather data and track users' behavior online. One area of confusion is around privacy settings for Adobe Flash, which can be difficult to understand and manage. While users can disable flash cookies using either the Global Storage Settings or Website Storage Settings elements of Adobe’s control panel, many users don’t understand how to do this, or don’t know that flash cookies are running, due to a default setting that fails to inform the user.
Another issue with companies using flash cookies on their websites is related to their relative permanence. Some websites are now reviving conventional cookies, and using the more tenacious flash cookies as markers, a process known in the IT world as re-spawning. This raises more questions about whether flash cookies should be regulated as potential tracking devices.