From WikiPilipinas: The Hip 'n Free Philippine Encyclopedia
A prejudicial question is understood in law as to be that which arises in a case the resolution of which is a logical antecedent of the issue involved in said case and the cognizance of which pertains to another tribunal. 
It is one based on a fact distinct and separate from the crime but so intimately connected with it that it determines the guilt or innocence of the accused, and for it to suspend the criminal action, it must appear not only that said case involves facts intimately related to those upon which the criminal prosecution would be based, but also that in the resolution of the issue or issues raised in the civil case, the guilt and innocence of the accused would necessarily be determined. 
The doctrine of prejudicial question comes into play generally in a situation where civil and criminal actions are pending and the issues involved in both cases are similar or so closely-related that an issue must be preemptively resolved in the civil case before the civil case may proceed. Thus, the existence of a prejudicial question in a civil case is alleged in the criminal case to cause the suspension of the latter pending final determination of the former. 
The essential elements of a prejudicial question as provided in Sec. 5, Rule 111 of the Revised Rules of Court are: (1) the civil action involves an issue in the criminal action; and (2) the resolution of such issue determines whether or not the criminal action may proceed. 
- ^ Quiambao vs. Osorio, GR 48157, 16 March 1988. Third Division, Fernan, J.
- ^ Donato vs. Luna, GR 53642, 15 April 1988. First Division, Gancayco, J.