Formally, LORETA is a particular 3D, discrete, distributed, linear solution to the inverse EEG/MEG problem. Informally, LORETA is described in the first published abstract:
The second published abstract reads:
several failed attempts, LORETA finally got published in a peer reviewed journal
Pascual-Marqui RD, Michel CM, Lehmann D. Low resolution electromagnetic tomography: a new method for localizing electrical activity in the brain. International Journal of Psychophysiology. 1994, 18:49-65.