It's a concept that is specific to Spanish grammar (and not even all Spanish linguists agree in this area) and Greek.

Voz media
En algunas lenguas, como la griega, existe una voz media con morfemas característicos para expresar acciones que realiza el sujeto en el interior de su conciencia sin que se manifiesten exteriormente para el interlocutor y sin que pueda concebirse un agente exterior. El español ha desarrollado una voz media para expresar las acciones inmanentes a base del morfema se, unido de tal forma al verbo, que sin él no tiene conjugación: atreverse, vanagloriarse, jactarse, esforzarse, arrepentirse, adherirse, acordarse, figurarse, asustarse, avergonzarse.
http://www.geocities.com/sergiozamorasin/voces.htm

He has published a variety of papers on reflexive, middle, impersonal and causative constructions in Spanish (1988, 1993, 1995, 1997, 2000, 2002), a language acquisition study of the Spanish reflexive and middle markers (with Donna Thal and Donna Jackson 1998) as well as a book on Spanish middle voice (A media voz 1999). His work on Tarascan focuses on middle voice and causatives (with Fernano Nava 2002, 2005).
http://www.uaq.mx/lenguas/maestria/docentes/maldonado.html

voice, grammatical category according to which an action is referred to as done by the subject (active, e.g., men shoot bears) or to the subject (passive, e.g., bears are shot by men). In Latin, voice is a category of inflection like mood or tense. In ancient Greek, verbs were conjugated in three voices: active, passive, and middle (reflexive).
http://reference.allrefer.com/encyclopedia/V/voice2.html