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triad tri·ad (trī'ād', -əd)
A collection of three things or symptoms having something in common.
The transverse tubule, and the terminal cisternae on each side of it, in a skeletal muscle fiber.
in chemistry, any of several sets of three chemically similar elements, the atomic weight of one of which is approximately equal to the mean of the atomic weights of the other two. Such triads-including chlorine-bromine-iodine, calcium-strontium-barium, and sulfur-selenium-tellurium-were noted by the German chemist J.W. Dobereiner between 1817 and 1829. The triad was the earliest atomic-weight classification of the elements