Viharas are permanent shelters built by monks and nuns. These are also referred to as monasteries. The first viharas were composed of wood, followed by brick. Today most viharas are made of concrete. The word vihara means "monastery" or "retreat". In ancient India, the term vihara was used to refer to any monastery for Buddhist monks. However, over time this definition was changed so that now includes only those temples that are not belonging to a larger monastery or complex. Thus, some buildings that were originally viharas but now operate as museums or offices are no longer considered viharas. In Buddhism, viharas are places where monks can stay in order to gain knowledge and insight into Dharma. They are not living quarters nor are they restaurants. Instead, viharas are places where monks can practice meditation, reflect on their experiences, and prepare themselves for future challenges. There are three types of viharas: main vihara, annex vihara, and adhyatma vihara. A main vihara is where the senior monk lives and conducts all-day meetings with other monks and teachers. It is usually a large structure with many rooms that can accommodate up to 20 or 30 monks.