Telepathy was the ability to hear a person's thoughts, or send messages through the use of the mind. This ability was granted through the use of the Psionic field. It is unknown if all species use this, as it cannot explain telekinesis.
Telepathic techniques and abilities
- Mind meld - A form a touch telepathy used by Vulcans in which individuals minds are linked and effectively become a single entity. (TOS episode: "Dagger of the Mind" et al.)
- Telepathic illusions - The Talosians, powerful telepaths are capable of producing telepathic illusions which can place affected individuals in whole new worlds, doing almost anything. They can also use this ability to manipulate perceptions of the real world, giving the image that something isn't damaged, for example. (TOS episode: "The Cage") The Aenar also appear to have this ability. (ENT episode: "The Aenar") The Arivne can also induce illusory scenarios, if subjects willingly accept them.
- Augmentation of strength - Spock ripped in half a centimeters-thick section of tritanium while under the focused telepathic power of several Arivne. (TOS novel: Planet of Judgment)
According to Star Trek science consultant and writer André Bormanis, telepathy within the Star Trek universe works via the "psionic field", the "medium" through which unspoken thoughts and feelings are communicated through space. Some humanoids can tap into this field through a kind of sense organ located in the brain (e.g. the paracortex). In the same manner that human eyes can sense portions of the electromagnetic field, telepaths can sense portions of the psionic field. (source)