echt adj., echt- prefix - real, true, pure, genuine, authentic, natural, etc.; the opposite of ersatz.
eigen- prefix from eigen, eigen- "own": proper, characteristic; used in technical terms in physics and mathematics, for example in eigenfrequency, eigenvector, eigenvalue, eigenfunction, eigenspace, eigenstate. Contrast with valrus's use of the word, which may be far superior.
ersatz n., adj. from Ersatz- "substitute": imitation or substitute (inferior); artificial; opposite of echt.
Also, ansatz n. from Ansatz "statement, formulation; beginning, start": a technical term used by mathematicians and especially by theoretical physicists to describe a solution to a problem which is guessed (usually with some free parameters).