A written question on a relevant issue, submitted by one of the parties in a civil proceeding to the other before the trial, requiring a written response. Interrogatories are part of the discovery process.
Written questions designed to discover key facts about an opposing party's case, that a party to a lawsuit asks an opposing party (but not a witness, who can only be questioned in person at a deposition). Interrogatories are part of the pretrial discovery stage of a lawsuit, and must be answered under penalty of perjury. Court rules tightly regulate how, when and how many interrogatories can be asked. Lawyers can write their own sets of questions, or can use form interrogatories, designed to cover typical issues in common lawsuits.