Find out the related word to complete the analogy.
$$\text{Triangle : Polygon :: Square : ?}$$

The relationship in the first pair is based on classification:
* A Triangle is a specific type of geometric figure that belongs to the broader class of Polygons (specifically, a three-sided polygon).

Applying the same logic to the second pair:

* A Square is a specific type of geometric figure that belongs to the broader class of four-sided polygons, which are called Quadrilaterals.

While a square is also a rectangle and a rhombus, the term "Polygon" represents the overarching classification category for a triangle. Therefore, the corresponding overarching classification category for a square is "Quadrilateral".