What is Flagrans Crimen?

Legal Definition
This, among the Romans, signified. that a crime was then or had just been committed for example, when a crime has just been committed and the corpus delictum is publicly exposed; or if a mob take place; or if a house be feloniously burned, these are severally flagrans cri men.

2. The term used in France is flagrant delit. The code of criminal instruction gives the following concise definition of it, art. "Le delit qui se commet actuellement ou qui vient de se coramettre, est un flagrant delit."
-- Bouviers Law Dictionary
Legal Definition
In Roman law. A fresh or recent crime. This term designated a crime in the very act of its commission, or while it was of recent occurrence
-- Black's Law Dictionary
Legal Definition
The crime being in the course of perpetration.
-- Ballentine's Law Dictionary