True name

A true name is a name of a thing or being that expresses, or is somehow identical to, its true nature. The notion that language, or some specific sacred language, refers to things by their true names has been central to philosophical study as well as various traditions of magic, religious invocation and mysticism (mantras) since antiquity.
Philosophical and religious contexts.
The true name of the Egyptian sun god Ra was revealed to Isis through an elaborate trick. This gave Isis complete power over Ra and allowed her to put her son Horus on the throne.
Socrates in Plato's "Cratylus" considers, without taking a position, the possibility whether names are "conventional" or "natural" natural being the "True name" ([τῇ ἀληθείᾳ ὄνομα]), that is, whether language is a system of arbitrary signs or whether words have an intrinsic relation to the things they signify (this anti-conventionalist position is called Cratylism).
Hellenistic Judaism emphasized the divine nature of "logos", later adopted by the Gospel of John. The true name of God plays a central role in Kabbalism (see Gematria, Temurah, YHWH [the tetragrammaton]) and to some extent in Sufism (see 100th name of God).
The ancient Jews considered God's true name so potent that its invocation conferred upon the speaker tremendous power over His creations. To prevent abuse of this power, as well as to avoid blasphemy, the name of God was always taboo, and increasingly disused so that by the time of Jesus their High Priest was supposedly the only individual who spoke it aloud — and then only in the Holy of Holies upon the Day of Atonement.
Also in a Biblical context, in the tale of Jacob's nocturnal wrestling with an anonymous angel, the angel refuses to reveal his own name to Jacob even after the angel's submission at dawn. Thereafter Jacob obtains a new name which signifies his successful struggle to God and man, and names the place to commemorate his surviving an encounter with the Divine.
Chinese Daoist traditions such as the Three Sovereigns corpus emphasize the capacity of talismans, charts, and diagrams to depict the true forms ("zhenxing" 真形) and true names ("zhenming" 真名) of demons and spirits. These talismanic representations are considered to be windows into the metaphysical substance and immutable essence of things — that is, images of the eternal Dao without form. The true form or name of a spirit inscribed on a talisman is legible only to supernatural beings, and gives a sort of temporary "control" over the entity whose name or form is possessed.
Contemporary pre-industrial peoples guard secret names which are only used in solemn rituals. These names are never mentioned and kept from general knowledge.
Folklore and literature.
In Jewish tradition, when several children have died in a family the next that is born has no name given to it, but is referred to as "Alter" (, literally "old"), or "Alterke", the view being that the Angel of Death, not knowing the name of the child, will not be able to seize it. When such a child attains the marriageable age, a new name, generally that of one of the Patriarchs, is given to it.
When captured by Polyphemus, Homer's Odysseus is careful not to reveal his name; when asked for it, Odysseus tells the giant that he is "Οὖτις", which means "nobody". But later, having escaped after blinding Polyphemus and thinking himself beyond Polyphemus' power, Odysseus boastfully reveals his real name, an act of hubris that was to cause enormous problems later. Knowing his name, Polyphemus was able to call upon Odysseus the revenge of his father, the god Poseidon. Many later episodes of the "Odyssey" depict Odysseus facing the relentless hostility of Poseidon—which he could have avoided had he persisted in keeping his real name secret. 
According to practises in folklore, referred to as 'the Law of Names'; knowledge of a true name allows one to affect another person or being magically. It is stated that knowing someone's, or something's, true name therefore gives the person (who knows the true name) power over them. This effect is used in many tales, such as in the German fairytale of "Rumpelstiltskin": within Rumpelstiltskin and all its variants, the girl can free herself from the power of a supernatural helper who demands her child by learning its name.
A legend of Saint Olaf recounts how a troll built a church for the saint at a fantastic speed and price, but the saint was able to free himself by learning the troll's name during a walk in the woods. Similarly, the belief that children who were not baptised at birth were in particular danger of having the fairies kidnap them and leave changelings in their place may stem from their unnamed state. In the Scandinavian variants of the ballad "Earl Brand", the hero can defeat all his enemies until the heroine, running away with him, pleads with him by name to spare her youngest brother.
In Scandinavian beliefs, more magical beasts, such as the Nix, could be defeated by calling their name. For the same reason significant objects in Germanic mythology, which were considered to have some kind of intrinsic personality, had their own names too, for example the legendary Sword Balmung.
In the folklore of Northern England, there was the belief that a boggart should never be named, for when the boggart was given a name, it could not be reasoned with nor persuaded, but would become uncontrollable and destructive.
Giacomo Puccini used a similar theme in the opera "Turandot". The plot turns on whether or not Princess Turandot could learn the name of her unwanted suitor. If she does, she could execute him; if she doesn't, she would have to marry him.
In cryptography.
The term "true name" is sometimes used in cryptography and computer security to refer to a name that is assumed to uniquely identify a principal in a global namespace (for example, an X.500 or X.509 Distinguished name). This usage is often critical, with the implication that use of true names is difficult to enforce and unwise to rely on. 
In popular culture and fiction.
In fantasy where magic works by evoking true names, characters often go to great lengths to conceal their true names. In some settings, such as Ursula K. Le Guin's "Earthsea", this is true for all beings. In others, as in Larry Niven's "The Magic Goes Away", it applies only to those of magical inclination, as where a wizard is revived from the dead only by another who found his name, and even then only with great difficulty. Finding a true name may require arcane procedures. In "Earthsea", a wizard must listen for and give the hero his true name; this is performed in both Le Guin's "A Wizard of Earthsea" and "The Tombs of Atuan".
A character remembering their true name may be an important means of maintaining mastery of their own life. In Hayao Miyazaki's movie "Spirited Away", the witch who runs the bathhouse, Yubaba, ensures loyalty by stealing the names of her subjects. For example, one of the witch's most loyal subjects, the spirit of the Kohaku River, has his name taken and is given a slave name: Haku. He forgets his name, and it is in this way 'taken' from him; he warns Chihiro Ogino against the dangers of forgetting her own name. She frees him when she recognises him and he then remembers and 'takes back' his name and is freed from the clutches of the witch.
In the cyberpunk genre following Vernor Vinge's 1981 "True Names" and the work of William Gibson, much of the plot involved interactions between people's virtual selves in cyberspace. Learning a fellow hacker's real-world name (i.e., their "true name") could allow you to turn them in to the government or otherwise blackmail them, conveying a kind of power that could be considered analogous to the equivalent concept of myth and legend.