In Spanish criminal procedure actor civil refers to a party to a criminal proceeding who is seeking compensation for damages for civil liability arising out of the criminal act being tried. In Anglo-American jurisdictions claims for compensation for damages arising from criminal offenses are typically filed in a separate civil suit and, thus, the role of the actor civil in the Spanish criminal process may often be unclear to English-speaking audiences. In brief, in Spain claims for compensation for damages arising during the commission of a crime (reclamación de la responsabilidad civil derivada de la comisión de un delito) are usually tried simultaneously with the prosecution of criminal defendants during criminal proceedings. Actor civil designates the party who enters an appearance in a criminal proceeding to file a claim for damages in what amounts to a civil liability action conducted simultaneously with (or within) a criminal procecution.
Thus, civil and criminal liability may be determined in the same proceeding if the actor civil enters an appearance and asks to be acknowledged as a party to the criminal proceedings as a plaintiff claiming damages based on the same incident for which a conviction for criminal liability is being sought (la parte solicita que se la tenga por personada en el procedimiento en calidad de actora civil). And in perhaps most cases the victim and and person seeking civil redress are the same.
Actor civil has often been rendered literally as “civil plaintiff” or “civil claimant,” translations that perhaps fail to reflect the fact that the expression actually denotes a party to a criminal proceeding. And, indeed, since this “civil action within a criminal trial” concept will perhaps be foreign to English-speaking audiences, actor civil may require a more detailed descriptive translation such as “party claiming civil damages in a criminal proceeding.” In other respects, the civil claim heard during a criminal trial is often referred to as a proceso civil acumulado, precisely because it is, in effect, a civil action joined or consolidated with (within) a criminal procecution.
Read more here: Víctor Moreno Catena and Valentín Cortés Domínguez. “El llamado ‘actor civil’ en el proceso penal.” Derecho Procesal Penal. Valencia: Tirant lo Blanch, 2005, p. 125 ff.