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pentad|ˈpɛntæd|
[ad. Gr. πεντάς, -άδα, later forms of πεµπάς, -άδα a group of five: see -ad 1 a.]
1. The number five (in the Pythagorean System); a group of five.
1653H. More Conject. Cabbal. (1713) 153 So manifest is it what special reason Pythagoras had to mention the Tetrad, rather than the Pentad, or any other number, in that form of swearing by Him that first imparted the Cabbala. 1660Stanley Hist. Philos. ix. (1701) 382/2 The Pentad is the first complexion of both kinds of number, even and odd, two and three. 1891Driver Introd. Lit. O. Test. (1892) 48 The laws appear often to be arranged in Pentads, or groups of five.
2. a. A period of five years. (Cf. decade 2.)
1880J. D. Whitney Climatic Changes vii. 337 The means of the last two pentads, 1866–70 and 1871–75, were almost exactly the same as the grand mean. 1978Nature 26 Jan. 322/2 South of lat 45° S, however, they conclude that average annual temperatures increased between the 1960–64 and 1970–74 pentads.
b. Meteorol. A period of five days.
1906W. Marriott Hints to Meteorol. Observers (ed. 6) 67/2 Pentad, a period of five days. 1935Nature 12 Oct. 614/1 There has been great discussion of the relative advantages of the 5-day period, or pentad, and the week, with the result that both units have received international approval. 1959R. E. Huschke Gloss. Meteorol. 416 Pentad, a group of five. In climatology, it is applied to a period of five consecutive days. It often is preferred to the week for climatological purposes since it is an exact factor of the 365-day year.
3. Chem. An element or radical that has the combining power of five units, i.e. of five atoms of hydrogen. Also attrib. or adj.
1877Watts Fownes' Chem. I. 460 Vanadium was, till lately, regarded as a hexad metal..; but Roscoe has shown that it is a pentad. 1880Athenæum 11 Dec. 781/3 The authors..conclude that in that substance phosphorus is a pentad.
Hence pentadic |-ˈædɪk| a., of the nature of a pentad (sense 3), pentavalent; whence pentadicity |-ˈdɪsɪtɪ|, the fact of being a pentad. |