A memory is a record of an event originating from a person's mind.
Extraction and external viewing
Memories can be extracted from a person's mind and reviewed, from an outside point of view, in a Pensieve. These memories appear as silver, hair-like wisps, and can be stored in bottles. Extracted memories can be altered, as Horace Slughorn attempted to do to a memory he gave to Albus Dumbledore in the 1996–1997 school year, although such alterations leave noticeable flaws in the recollection — this particular memory left sudden clouds with a booming voice in place of the crucial scenes; Dumbledore noted it was crudely done so if done with more details and skills, a false memory may not be flawed. A fragment of Lord Voldemort's soul was able to show his memories through the power of his diary, rather than the need of extracting it and placing it into a Pensieve.
Once viewing the memory, the viewers cannot be seen or heard by anyone other than other viewers. What truly makes this phenomena remarkable is the fact that viewers can witness every aspect of the memory as they see fit from a third person perspective, even if the owner of the memory has little to no knowledge of what they are seeing, since one viewer was able to tail other people in the memory whilst the past version of the memory's owner was present at a fair distance away. The viewer is also intangible to all other physical matters and energies of the said memory, as Harry Potter was considered a phantom, invisible and intangible, to the rest of the past world he was viewing; a man shaking hands with Dumbledore went right through Harry's body, causing neither of them harm, and a later memory when Harry was phasing through many desks in the Great Hall while rushing to his past father's location, to which was described if done in the real world the desks would be sent flying.
Memories can be read without the needs of extracting them, through the powers of Legilimency; however, those that were extracted and then deposited into a magical device like the Pensieve or a simple vial can no longer be viewed through this manner. Also, Veritaserum works based on the memories of the person, and if there are any alterations done to such memories, they would speak what they assume to be the truth, not the truth that has been suppressed. Occlumency, the art of suppressing such memories and thoughts, is a way to counter the two aforementioned methods. A master Occlumens such as Severus Snape can choose what memories to suppress instead of completely blocking out Legilimency, so as to prevent the opposition from knowing crucial information without knowing that they are blocking it out on purpose.
External interference and damage
Memories can receive external interferences such as the Memory Charm or False Memory Charm, which can suppress certain memories or create fake ones, respectively. Such alterations may cause permanent memory damage if done at great intensity or incorrectly, as Gilderoy Lockhart and Bertha Jorkins both suffered severe amnesia that was irreversible. When Lord Voldemort forcibly broke through the Memory Charm on the latter, she remembered all of the things that were suppressed, but at the price of her body and mind suffered damage far beyond repair. Such alterations can also be bypassed such as when Dumbledore extracted a true memory over a false one in Morfin Gaunt, though he noted it to be a difficult task. In a related matter, one's memories may be improved through a Memory Potion, Baruffio's Brain Elixir and powdered dragon claw.
- Tom Riddle's memory that he showed Harry Potter regarding the opening of the Chamber of Secrets in 1993. This was shown through the power of a diary horcrux.
- Albus Dumbledore's memory of the trial and sentencing of Barty Crouch Jr. and the Lestranges. Viewed by Harry Potter in 1995.
- Albus Dumbledore's memory of Bertha Jorkins
- Harry Potter's various memories. Viewed by Severus Snape through legilimency in 1996.
- Severus Snape's memory of when he was being bullied by the Marauders in his fifth year. Viewed by Harry Potter in 1996.
- Albus Dumbledore's memory of Sybill Trelawney's Prophecy concerning Harry Potter and Voldemort.
- Bob Ogdens memory of the Gaunts. Viewed by Harry Potter and Albus Dumbledore in 1996.
- Albus Dumbledore's memory of meeting the 11 year old Tom Riddle. Viewed by Harry Potter and Albus Dumbledore in 1996.
- Morfin Gaunt's memory of meeting the teenage Tom Riddle for the first time. Viewed by Harry Potter and Albus Dumbledore in 1997.
- Hokey the House-elf's memory of Hepzibah Smith and Tom Riddle. Viewed by Harry Potter and Albus Dumbledore in 1997.
- Albus Dumbledore's memory of Tom Riddle's job interview for a teaching spot at Hogwarts. Viewed by Harry Potter and Albus Dumbledore in 1997.
- Horace Slughorn's memory of when he explained horcruxes to Tom Riddle, first crudely modified and then the real, undiluted one. Viewed by Harry Potter and Albus Dumbledore in 1997.
- Severus Snape's memories regarding himself and Lily Potter. Viewed by Harry Potter in 1998.
- Multiple exchanges between the minds of Harry Potter and Lord Voldemort.
- Altheda in The Tales of Beedle the Bard cast the memories of her lost lover into a stream.
- Harry Potter and the Chamber of Secrets (First mentioned)
- Harry Potter and the Chamber of Secrets (film)
- Harry Potter and the Chamber of Secrets (video game)
- Harry Potter and the Goblet of Fire (First appearance)
- Harry Potter and the Goblet of Fire (film)
- Harry Potter and the Order of the Phoenix
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Half-Blood Prince (film)
- Harry Potter and the Half-Blood Prince (video game) (Both real and IPhone version)
- Harry Potter and the Deathly Hallows
- Harry Potter and the Deathly Hallows: Part 2
- Harry Potter and the Deathly Hallows: Part 2 (video game)
- The Tales of Beedle the Bard
- Harry Potter: The Character Vault