Definition of dyad :
1. An element, atom, or radical having a valence or combining power of two.
2. 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.
3. Two units treated as one; a couple; a pair.
duality (part of speech: noun)
microorganism (part of speech: noun)