Encyclopaedia Britannica, 11th Edition, "Diameter" to "Dinarchus" / Volume 8, Slice 4

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DIAMETER (from the Gr. διά, through, μέτρον, measure), in geometry, a line passing through the centre of a circle or conic section and terminated by the curve; the “principal diameters” of the ellipse and hyperbola coincide with the “axes” and are at right angles; “conjugate diameters” are such that each bisects chords parallel to the other. The diameter of a quadric surface is a line at the extremities of which the tangent planes are parallel. Newton defined the diameter of a curve of any order as the locus of the centres of the mean distances of the points of intersection of a system of parallel chords with the curve; this locus may be shown to be a straight line. The word is also used as a unit of linear measurement of the magnifying power of a lens or microscope.
The name Άδάμας, “the invincible,” was probably applied by the Greeks to hard metals, and thence to corundum (emery) and other hard stones. According to Charles William King, the first undoubted application of the name to the diamond is found in Manilius (a.d. 16),— Sic Adamas , punctum lapidis , pretiosior auro ,—and Pliny (a.d. 100) speaks of the rarity of the stone, “the most valuable of gems, known only to kings.” Pliny described six varieties, among which the Indian, having six pointed angles, and also resembling two pyramids ( turbines , whip-tops) placed base to base, may probably be identified as the ordinary octahedral crystal (fig. 1). The “diamond” ( Yahalom ) in the breastplate of the high priest (Ex. xxxix. 11) was certainly some other stone, for it bore the name of a tribe, and methods of engraving the true diamond cannot have been known so early. The stone can hardly have become familiar to the Romans until introduced from India, where it was probably mined at a very early period. But one or other of the remaining varieties mentioned by Pliny (the Macedonian, the Arabian, the Cyprian, &c.) may be the true diamond, which was in great request for the tool of the gem-engraver. Later Roman authors mentioned various rivers in India as yielding the Adamas among their sands. The name Adamas became corrupted into the forms adamant , diamaunt , diamant , diamond ; but the same word, owing to a medieval misinterpretation which derived it from adamare (compare the French word aimant ), was also applied to the lodestone.