"Fear names. Names have power in identity. Others can use names as weapons. Names are a hook that can be used to track you... Remain nameless, and you shall be safe.
I am The Nameless One."
Only I know your true purpose in this world, Lombax. Only I know your true name!
— Emperor Tachyon, Ratchet & Clank Future: Tools of Destruction
(a Macbeth-esque witch/Carrionite alien howls in pain/rage and vanishes)
Martha: What did you do?
The Doctor: I named her. The power of a name. That's old magic.
Rose:I know your true name, Baba Yaga. Do you wish me to shout it now, so that all of the animals of the forest, all the birds of the air, every passing nixie and boggart will know it too? Your name will be as common as crab-grass. Would you like that, Baba Yaga?
Baba Yaga: You're lying. You do not know my name.
Rose: Perhaps. Do you wish to find out how loudly I can shout?
— The Books of Magic, "Land of Summer's Twilight"
You know enough not to look in my eyes, but you do not know not to ask me my name?
Names, capital N, have power. If you know something's name, you automatically have a conduit with which you can reach out and touch it, a way to home in on it with magic. Sometimes this can be a really bad idea. Speak the Name of a big, bad, spiritual entity and you might be able to touch it, sure—but it can touch you right back, and the big boys tend to do it a lot harder than any mortal.
—Harry Dresden, Small Favor
The name is the thing, and the true name is the true thing.
Look, my name, my "real name", that is not the point. The name I chose is the Doctor. The name you choose, it's like a promise you make.
— The Eleventh Doctor