Final answer:
Dehydration is the serious condition when a person lacks enough fluid in the body. It can occur due to various factors and leads to symptoms such as thirst, dizziness, and fatigue.
Step-by-step explanation:
Dehydration refers to water loss from the body without adequate replacement. It can be caused by factors such as prolonged physical activity without enough water intake, heat exposure, excessive weight loss, vomiting, diarrhea, and more. Physiologically, dehydration decreases blood volume and can lead to symptoms such as thirst, dizziness, fatigue, and decreased urine output.