Telepathy (テレパシー) is a way to communicate with others using one's mind. Users can also read the minds of the ones they communicate with, similar to telepathic eavesdropping. Several of characters have demonstrated this ability throughout the Dragon Ball series.
Telepathy was first used in the series when Master Roshi reads the mind of Nam during the 21st World Martial Arts Tournament, thus learning of Nam's desperate search for water. It is used by members of Master Shen's Crane School to communicate with each other during the 22nd World Martial Arts Tournament, and then by King Piccolo to communicate with his offspring.
King Kai is possibly one of the most famous users of this technique, as he is able to speak with people all across the universe, and can even speak to several people at one time. It can be assumed that he taught it to Goku while he trained with him in Other World, as Goku uses this to stop Krillin from finishing Vegeta after their battle on Earth. Piccolo uses telepathy several times to communicate with Gohan. It also used by Supreme Kai to read the mind of Piccolo while the two are facing each other at the 25th World Martial Arts Tournament.
The ability to communicate telepathically does not seem to be hindered by time dilation, as Piccolo is able to talk to Goten and Trunks while they are in the Hyperbolic Time Chamber during the Fusion Saga, despite him being outside of it.