Dichotomy means a division into two mutually exclusive, exhaustive, or contrasting parts. The word comes from the Greek dikhotomia ("cutting in two").

1. Logic / Philosophy A dichotomy splits a domain into exactly two categories with no overlap and no gap - e.g., true / false, finite / infinite. A false dichotomy (false dilemma) is a fallacy where a situation is presented as having only two options when more exist.

2. Everyday Language Often used loosely to mean a sharp contrast or tension between two things:

3. Biology Branching into two equal parts - e.g., dichotomous branching in plants or classification keys.

4. Astronomy The phase of the Moon or an inferior planet (Mercury, Venus) when exactly half of the visible disk is illuminated - the moment of "half moon."

5. Dichotomous Key (Science) A step-by-step identification tool that presents two mutually exclusive choices at each step to classify organisms or objects.

This ignores a third option (neutrality, partial agreement, etc.). Recognizing false dichotomies is a core critical thinking skill.

Let me know if you want a deeper look at any specific context - logical, biological, philosophical, or otherwise.