Which term best describes the entire outer boundary line of a fire's spread?

The entire outer boundary line of a fire's spread is best described as the fire perimeter. This term refers to the perimeter that encompasses all areas affected by the fire and indicates where the actively burning area meets unburned fuel. Understanding the concept of fire perimeter is crucial for fire management and control operations, as it helps in mapping the extent of the fire, assessing threats to life and property, and strategizing containment efforts.

In contrast, the other terms describe specific characteristics or formations related to the behavior of wildland fires rather than the fire's overall boundary. An island refers to a portion of unburned fuel completely surrounded by burnt area, pockets of a fire are small areas of the fire that may burn up or down but do not represent the overall extent, and fingers of a fire denote elongated projections of flame that extend into unburned fuel but are not the outer boundary as a whole.