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I would like to know what to call the part of the definition that is defined and the part that defines. For an explanation this would be explanandum and explanans.

In the following example the concept of a planet is defined:

A planet is an astronomical object orbiting a star or stellar remnant that

  • is massive enough to be rounded by its own gravity,
  • is not massive enough to cause thermonuclear fusion, and
  • has cleared its neighbouring region of planetesimals.

This is taken out of wikipedia ( https://en.wikipedia.org/wiki/Planet ). I'm aware of the vagueness of this particular definition and the quarrels that took place around the status of Pluto. But I'm interested here in terminology.

What would be the term that describes the role of "Planet" in this definition?

5

The expression being defined is called the definiendum of the definition, and the expression that defines is called the definiens.

In your example 'Planet' would be the definiendum.

Reference

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