Dosha is a Sanskrit word meaning the one which can pollute the body, however all three are needed to create balance in the body. If one is too low or too high this produces imbalance. So this aggravation is called Vikriti and the balanced, innate state is called Prakriti.
Ayurveda describes three fundamental mind/body types, or doshas, called Vata, Pitta and Kapha, which embody different combinations of the five elements : air, ether, fire, water and earth. Each of us contain all doshas to varying degrees, and their balance can be determined through examination of your eyes, nails, tongue, skin, voice, by pulse diagnosis, and through the insight of the Ayurvedic practitioner. Because the five elements exist in us and in nature, your doshas can become aggravated or imbalanced due to inappropriate or inadequate diet, change of seasons, climate or lifestyle, and result in disease in the body.