Thirst is the desire to drink something. It can be triggered by the loss of fluid volume in and around cells and in the blood. Thirst is your body's way of saying you need water to avoid dehydration.
Thirst has a behavioral component as well and can be triggered by aromas and flavors, so just thinking about your favorite beverage can make you thirsty. It's also important to note that older people often have problems with the thirst mechanism and may not feel thirsty even when they're dehydrated.