The earliest of the four phases of a fire. During this phase, the products of combustion may be minimal, the changes in the surrounding atmosphere may be difficult to observe (only some smoke, no detectable flame), and the amount of heat generated will not significantly affect the surrounding area. The incipient phase can last a few moments (i.e., ignition of a combustible liquid), hours, or even days (i.e., the exothermic reaction seen in spontaneous combustion).
Redsicker, David R. and O’Connor, John J. Practical Fire & Arson Investigation (Second Edition). CRC Press. 1997.
Icove, David J and DeHaan, John D. Forensic Fire Scene Reconstruction. Pearson Prentice Hall. 2004.