[ad. Gr. τετράς (τετραδ-) a group of four, the number four.]
1. A sum, group, or set of four; four (things, etc.) regarded as a single object of thought.
1653. H. More, Conject. Cabbal. (1713), 82. It was a solemn Oath … to swear by him that delivered to them the mystery of the Tetractys, Tetrad, or number Four. Ibid. [see TETRACTYS].
1832. Coleridge, Table Talk, 24 April. The adorable tetractys, or tetrad, is the formula of God.
1895. Athenæum, 2 Feb., 151/1. The great tetrad of senior wranglers of 1840 to 1843
2. In spec. uses. a. Chem. An element, compound, or radical having a combining power of four units, i.e., of four atoms of hydrogen; a tetravalent element, etc.
1865. Reader, 1 April, 372/3. A tetratomic atom or tetrad.
1866. Roscoe, Elem. Chem., xxvii. 242. As in mineral chemistry we have radicals some of which are monads, and some dyads, triads, or tetrads.
1868. Fownes’ Chem. (ed. 10), 259. Silicium and titanium are tetrads.
b. Biol. (a) A group of four cells, e.g., spores, pollen-grains. (b) A group of four chromosomes formed by the division of a single chromosome. (c) A quaternary unit of organization differentiated from a triad.
1876. trans. Schüzenberger’s Ferment., 52. In the tetrads arranged in the form of a cross, we observe, also, two plane surfaces at right angles.
1882. Vines, Sachs’ Bot., 456. The cavity of the sporangium becomes filled with a granular plasma in which lie the mother-cells and the tetrads of spores…. All the spores of the sixteen tetrads formed in the microsporangia reach maturity.
1883. [see 3].
1895. Oliver, trans. Kerner’s Nat. Hist. Plants, II. 101. In Rhododendron hirsutum all the pollen-tetrads of an anther-cavity are held together by a mass of sticky viscin.
a. 1909. (in sense b) Wilson (cited in C. D., Suppl.)
1909. J. W. Jenkinson, Exper. Embryol., 108. Granules of chromatin took the place of the tetrads and were unequally distributed to the spindle poles.
c. Mus. A chord of four notes (after TRIAD).
1881. Broadhouse, Mus. Acoustics, 332. The great majority of major tetrads in Palestrina’s Stabat Mater are in the positions 1, 10, 8, 5, 3, 2, 4, 9.
d. In ancient systems of arithmetical notation: A group or series of four characters corresponding to successive powers of ten.
1883. Sir E. C. Bayley, Geneal. Mod. Numerals, II. 90. They [the Greeks] had however a system of ‘octads’ and ‘tetrads’ for expressing numbers of very high value.
6. Math. (See quot.)
1889. Cayley, Math. Papers, XII. 590. The term ‘tetrad’ is used in two distinct … senses, viz. a tetrad denotes any four points; and it also denotes the four vertices of a self-conjugate tetrahedron in regard to a quadric surface…. Two or more tetrads, in regard to one and the same quadric surface, are called similar tetrads.
3. attrib., as tetrad metal, term; tetrad-deme Biol., an aggregation of tetrads: see 2 b (b) and DEME2 2.
1866. Odling, Anim. Chem., 17. The fourth or tetrad term of our series of typical hydrides.
1868. Fownes’ Chem. (ed. 10), 445. Tin is a tetrad metal.
1883. P. Geddes, in Encycl. Brit., XVI. 843/2. Starting from the unit of the first order, the plastid or monad, and terming any undifferentiated aggregate a deme, we have a monad-deme integrating into a secondary unit or dyad, this rising through dyad-demes into a triad, this forming triad-demes, and these when differentiated becoming tetrads, the Botryllus-colony with which the evolution of compound individuality terminates being a tetrad-deme.
