Verified Answer :

The correct answer is:

line.

Explanation:

In Euclidean geometry, there are 3 terms that are considered "undefined":  point, line and plane. 

They are considered undefined because they are described, but not every formally defined.

The other terms in this question, pyramid, square and triangle, are all formally defined.

Answer: Consider the terms pyramid, line, square, and triangle. The term  line is not defined in Euclidean geometry.

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