Among the most common terms today, śastra-vidyā, is a compound of the words ' (weapon) and ' (knowledge). Dhanurveda derives from the words for bow (') and knowledge ('), the science of archery in Puranic literature, later applied to martial arts in general. The Vishnu Purana text describes dhanuveda as one of the traditional eighteen branches of applied knowledge or upaveda, along with shastrashastra or military science. A later term, yuddha kalā, comes from the words yuddha meaning fight or combat and kalā meaning art or skill. The related term śastra kalā (lit. weapon art) usually refers specifically to armed disciplines. Another term, yuddha-vidyā or combat knowledge, refers to the skills used on the battlefield, encompassing not only actual fighting but also battle formations and strategy. Martial arts are usually learnt and practiced in the traditional akharas.