n. 二, 一对, 二元一位

[化] 二价元集(或基); 双; 对; 成对之物; 并矢

[医] 二价元素, 二分体

[ noun ]
two items of the same kind
<noun.quantity>

Dyad \Dy"ad\, n. [L. dyas, dyadis, the number two. Gr. ?: cf. F.
dyade. See two, and cf. {Duad}.]
1. Two units treated as one; a couple; a pair.

2. (Chem.) An element, atom, or radical having a valence or
combining power of two.


Dyad \Dy"ad\, a. (Chem.)
Having a valence or combining power of two; capable of being
substituted for, combined with, or replaced by, two atoms of
hydrogen; as, oxygen and calcium are dyad elements. See
{Valence}.