noun

• (morphology of segmented Metazoa) A specialized, structured, grouping of functionally related segments, or analogous subunits, into units such as the head, the thorax, and the abdomen, particularly in various Arthropoda and Vertebrata.
  "This group of four neuromeres constituting the procephalic lobes is further marked off from the rest of the plate by its height, and also by its darker color as in previous stages. These distinctive characters make it convenient to apply the term "tagma" suggested by Lankester for a more or less isolated and independent group of segments. The aptness of this term will become more apparent as the development of the procephalic lobes is followed in the succeeding stages and when it is discussed in the conclusion."
• (physiology, dated) A specialized, structured, grouping of functionally related molecules, forming the basis the physiology and structure of larger units such as cells and tissues.
  "1889 Prof. J. S. Burdon Sanderson: Elementary Problems in Physiology. In: Annual Report of the Board of Regents of the Smithsonian Institution, 1890. His view is that certain well-known properties of organized bodies require for their explanation the admission that the simplest visible structure is itself made up of an arrangement of units of a far inferior order of minuteness. It is these hypothetical units that Nägeli has called Micellae. Now, Nägeli, in the first instance, confounded the micella with molecules, conceiving that the molecule of living matter must be of enormous size. But inasmuch as we have no reason for believing that any form of living material is chemically homogeneous, it was soon recognized, perhaps first by Pfeffer, but eventually also by Nägeli himself, that a micella, the ultimate element of living material, is not equivalent to a molecule, however big or complex, but must rather be an arrangement or phalanx of molecules of different kinds. Hence the word Tagma, first used by Pfeffer, has come to be accepted as best expressing the notion. And here it must be noted that each of the physiologists to whom reference has been made, regards the micella, not as a mere aggregate of separate particles, but as connected together so as to form a system..."
• (historical) A military unit of battalion or regiment size, in the Byzantine empire of the 8th - 11th centuries.