Which of the following terms in mathematics is defined?

Options

A. Plane
B. Point
C. Line
D. Diagonal of a quadrilateral

Answer

To solve the question "Which of the following terms in mathematics is defined?" we need to analyze each option provided:

1. **Option A: Plane**
- A plane is a flat, two-dimensional surface that extends infinitely in all directions. It is defined in geometry as having length and width but no thickness. However, it is often described in terms of points and lines that lie on it.

2. **Option B: Point**
- A point is a fundamental concept in geometry that represents a location in space. It has no dimensions (length, width, or height) and is often represented by a dot. While a point is a basic unit in geometry, it is not defined in terms of other geometric entities.

3. **Option C: Line**
- A line is defined as a straight one-dimensional figure that has no thickness and extends infinitely in both directions. Like points and planes, lines are basic geometric concepts but are not defined in terms of other entities.

4. **Option D: Diagonal of a Quadrilateral**
- A diagonal is a line segment that connects two non-adjacent vertices of a polygon. In the case of a quadrilateral, it can be defined as a line segment that connects two opposite corners. This term is specific and can be defined based on the properties of quadrilaterals.

After analyzing each option, we can conclude that:

- **Correct Answer: Option D (Diagonal of a Quadrilateral)** is the term that is defined, as it is specific to the properties of quadrilaterals.

### Step-by-Step Solution:
1. **Identify the options**: List the terms provided in the question.
2. **Analyze each term**: Determine if each term can be defined in mathematical terms.
3. **Compare definitions**: Look for specificity in the definitions of the terms.
4. **Select the defined term**: Choose the term that has a clear definition based on other geometric properties.