A true name, to my limited understanding, is a concept of a name(s)/phrase specific to one person/being that gives the speaker power over the person the true name belongs to. It is a semi-popular concept in media, but not often used. I was wonder which cultures/Mythologies this came from.
The secret name of Ra
Ra was known by many names by gods and their worshipers, but one name of Ra was not spoken of since the beginning of time. This secret name of Ra was secret for a reason, To know this name would give the orator power over Ra and everything he created.
Isis was a very powerful goddess, Mistress of Magic and very wise. Wise enough to know that nothing in creation could harm its own creator. She devised a plan to turn Ra's power against him and formulated a devious plan.
Every day the Sun god walked through his kingdom, but even Ra was growing noticeably older. One day when Isis was mingling with the other gods and goddesses, she noticed a drop of saliva falling from Ra's mouth. As soon as nobody was watching she retrieved this drop and the earth that contained it.
As soon as it was dark she formed the earth and saliva into a clay serpent and used her spells on the serpent. She then placed this serpent in the grass on a crossroads the Sun King always took on his daily route through his kingdom.
The plan works
As Ra walked past the crossroads the serpent sprang to life and bit him in the ankle and then crumbled back into earth. Ra screamed so loud from the pain all of creation could hear him. Ra shook as the poison took hold of him. 'I have been wounded by something deadly that I did not create'. Ra didn't suspect his offspring and loved ones but was in great pain.
He sent forth messengers to all the Ennead so their magic could help him. Shu, Tefenet, Geb, Nut, Set, Osiris, Isis and Nepthys. Many more of his created deities where summoned to their creator. When everyone was summoned Isis came forth and offered to help him.
Father of All, she began, 'whatever is the matter'? Has some snake bitten you? Has some wretched creature dared to strike at the creator? Few of the gods can compare with me in their wisdom and I am the Mistress of Magic. If you let me help you, 'I'm sure I can cure you'. Harris, Geraldine (1981). Gods & Pharaohs from Egyptian Mythology page 25
Then, a grateful Ra told Isis everything that had happened that day.
Isis replied: Tell me your full name so I can use it in my spells. Without this knowledge my greatest magic cannot save you. After Ra told all his known names Isis asked for the only name Ra alone knew.
Ra moaned: My secret name was given to me so I can live at ease, and fear no living creature. How can I give it away?
Isis waited for the pain to be unbearable and Ra finally could not handle the excruciating pain. Ra ordered all other gods to stand back while he whispered his secret name to Isis. "Now the power of the secret name has passed from my heart to your heart, in time you can give it to your son (Horus) but warn him to never betray the secret".
Isis used her great magic to drive the poison out of Ra and knew that one day her son would wield the power of Ra and sit on the throne of Egypt.
It's hard to think of a culture which did NOT have the concept of a secret name in the past. Even today it is common as recently as the last generation who are still with us, do not refer to their spouse by name even with extended faamily - it is always Himself/Herself. Similarly a woman will refer to her daughters-in-law by their maiden names. This is not just about Names having power but WORDS in general - eg Gaelic, Welsh, Arabic, Greek and many other languages rarely say 'yes' or 'no' if a circumlocution can avoid doing so.