dichotomy

n. separation into two parts

Traditionally Dichotomy is any splitting of a whole into exactly two non-overlapping parts. This covers the symmetric form of dichotomy - but there is also the asymmetric form covered in bifurcation where one element has emerged from the other. This is NOT restricted to botany - it in fact covers the properties of the human brain as it aquires information by self-referencing the asymmetric dichotomy of differentiate/integrate.
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Noun
1. being twofold; a classification into two opposed parts or subclasses; "the dichotomy between eastern and western culture"
(synonym) duality
(hypernym) classification, categorization, categorisation
(derivation) dichotomize, dichotomise

(n.)
The place where a stem or vein is forked.
  

(n.)
That phase of the moon in which it appears bisected, or shows only half its disk, as at the quadratures.
  

(n.)
Successive division and subdivision, as of a stem of a plant or a vein of the body, into two parts as it proceeds from its origin; successive bifurcation.
  

(n.)
Division or distribution of genera into two species; division into two subordinate parts.
  

(n.)
Division into two; especially, the division of a class into two subclasses opposed to each other by contradiction, as the division of the term man into white and not white.
  

(n.)
A cutting in two; a division.
  

Synonyms and related words:
Janus, abscission, ambiguity, ambivalence, amputation, bifidity, biformity, bifurcation, bipartition, bisection, branching, butchering, by two, chopping, cleavage, conjugation, cutting, cutting in two, dimidiation, division, doubleness, doublethink, doubling, dualism, duality, duplexity, duplication, duplicity, enucleation, equivocality, excision, fission, forking, halving, in half, irony, laceration, mutilation, pairing, polarity, ramification, rending, resection, ripping, scission, section, severance, slashing, slicing, splitting, subdivision, surgery, tearing, twinning, two-facedness, twoness,