What is the definition or description of: heat exhaustion?

Excessive heat: Heat exhaustion is one of the conditions along the spectrum of heat illness. It occurs when the body's core body temperature exceeds 98.6 degrees f, which presents in symptoms such as excessive sweating and fatigue. If left unchecked it can lead to heat stroke, which can be life threatening.

1st phase heat strok: Heat exhaustion is the first phase of heat illness ending with heat stroke. When we exercise we generate heat that has to be lost though sweating (why hydration is important) and other mechanisms. As body temp starts to rise, fatigue, headache, confusion are symptoms of heat illness. Immediate rest and cooling are needed to prevent progression of heat illness at this stage.