The Angle-Angle (AA) similarity criterion states that if two angles of one triangle are equal to two angles of another triangle, then the two triangles are similar. This means that their corresponding sides are in proportion, and their shapes are the same, even if their sizes differ. This criterion is useful in geometry because it allows for the determination of triangle similarity without needing to know the lengths of the sides. It is often applied in various fields, including architecture, engineering, and art, where understanding proportions and shapes is essential.