Regular

A square is a polygon with all sides of equal length and all angles of equal measure (90 degrees). Such polygons are classified as "regular" polygons.

Let's analyze each option provided:

- Regular: A regular polygon has all sides of equal length and all angles of equal measure. Since a square meets these criteria, it is a regular polygon.

- Irregular: An irregular polygon does not have all sides of equal length and all angles of equal measure. This does not describe a square.

- Isosceles: Isosceles usually refers to a triangle with two sides of equal length. This term does not apply to polygons in general or specifically to squares.

- Concave: A concave polygon has at least one angle greater than 180 degrees, and it "caves in" at some point. A square does not have any angles greater than 180 degrees, so it is not concave.

The correct answer is:

"A square is considered a regular polygon."