Dehydration is a medical condition that occurs when the body loses more water than it takes in, resulting in an excessive loss of water from the body. It can be caused by not drinking enough fluids, sweating too much, or having diarrhea or vomiting. Symptoms of dehydration include feeling thirsty, dizziness, fatigue, dry mouth, and dark-colored urine. If left untreated, dehydration can lead to serious health complications.