Tulpa

A tulpa is a creature created by the sheer willpower/belief/generic-energy of an individual or individuals.

Gonna have to expand on that, huh?

Think of it this way. If you're reading a book or watching a TV show where Santa Claus exists because all the little kids in the world think he exists, Santa Claus -in that work- is a tulpa. If a little kid has an imaginary friend (or perhaps a stuffed animal) that they swear up and down is alive, then things start to happen that would insinuate they're telling the truth, then it's probably a tulpa.

Loads of works play with this idea, from Discworld (where belief is actually a measurable force in the universe), to some of Charles de Lint's stuff, to (some would argue) real life.

For you TvTropers, this is basically YourMindMakesItReal, GodsNeedPrayerBadly, and ClapYourHandsIfYouBelieve. (Warning, TvTropes link. It WILL eat up about an hour of your time, at the very least).

It's an interesting concept that's been done a zillion times over, yet still remains pretty cool. Even gets a little shoutout in the Bible, if you're interested in that kind of thing.

So if you don't get anything from Santa this year, remember, it's your own damned fault for doubting the fat man.

See also: egrigor. Maybe golem, too.
Apparently, the word itself derives loosely from the Sanskrit word "to build" or "to create".

Please keep in mind while reading this that I am no expert on this subject, nor do I have a Tulpa myself, or have any intention to create one. If you want to learn more, your can refer to the Tulpa Website or the Tulpa Subreddit

A Tulpa is a being that exists in someones mind, like an imaginary friend, that has its own thoughts and emotions. People create these beings usually to act as companions or advisers, and are either visualized in the mind's eye, or seen as a hallucination. The goal of creating a Tulpa may be compared to attempting to give oneself schizophrenia or multiple personality disorder, but 'hosts' of Tulpas insist that this is a healthy process that does not impede your body or mind. The process of creating a Tulpa involves creating a base form, or what the Tulpa will look like, then creating a personality around that base with meditation and concentration. Eventually you may ask your self (or what is forming into your Tulpa) questions and getting an answer that you didn't just think of. Through more concentration and meditation your Tulpa should begin to form into another consciousness independent of your own. Another part of having a Tulpa is a mindscape, or wonderland. This is a place in your mind where you and your Tulpa can interact visually without the host imposing their Tulpa in the physical world.