A thoughtform is a manifestation of mental energy, also known as a 'tulpa' in Tibetan mysticism. The concept of thoughtforms is related to the Western philosophy and practice of magick.
Thoughtforms are also known as egregores. The most famous thoughtform is Golem. The definition of Golem in Wikipedia is as follows:
In Jewish folklore, a golem (גולם, sometimes, as in Yiddish, pronounced goilem) is an animated being created entirely from inanimate matter. In modern Hebrew the word golem literally means "cocoon", but can also mean "fool", "silly", or even "stupid". The name appears to derive from the word gelem (גלם), which means "raw material".
Imaginary friends, demons, genies (islamic hawass), ghosts may be considered as thoughtforms too. In my humble opinion, I think that many Gods and Goddesses can be considered as thoughtforms too.
Below are a four part video series by Rob Cutter. He explains the creation methods of thoughtforms and he also tells his own story about it. In his experience, one night he saw his creation manifest into physical reality.