Several Buddhist terms and concepts lack direct translations into English that cover the breadth of the original term. In this list, the terms are in languages most relevant to their traditions, that is: English for terms that are common to all traditions. Pali for Theravada, Sanskrit for mainstream Mahayana, Tibetan for Vajrayana and Japanese for Zen. A short translation and a link to a (hopefully) more in-depth article is given, as well as the relevant tradition(s).
ajahn - "teacher" in the Thai language derived from the (Pali) word acarya. In the Theravada tradition it is generally a title given to a Bhikkhu or Bhikkhuni who has been fully ordained for ten rain retreats (10 years).
koan/gongan - Lit. "Public case." A meditative method developed in the Chan/Seon/Zen traditions, generally consisting of a problem that defies solution by means of rational thought
kyosaku - stick used to strike zazen practitioners in order to improve concentration (Japanese)
Jhana/Dhyana] - A state of meditative contemplation. See also: shamata, samadhi, samapatti.(Pali/Sanskrit)
Tathagata - The "Thus-Come One"; One of the Buddha's ten epithets
tipitaka/tripitaka - The "Three Baskets"; canon containing the sacred texts for Buddhism (Pali)
Tulku - A re-incarnated Tibetan teacher
vinaya - The monastic rules of discipline for Buddhist monks & nuns. (Sanskrit, Pali)
vipassana/vipashana - Insight meditation practiced in the Theravada tradition. In the tipitaka (Pali) it was stressed by the Buddha to be practiced in conjunction with Samatha.
See the Digital Dictionary of Buddhism for a list of over 29,000 Buddhist terms, compiled by a professional scholar of Buddhist Studies (log in with the user ID "guest" on the search engine)