1. Given that quadrilateral $A B C D$ is inscribed in $\odot O$, and $A D$ $/ / B C$. Then the shape of quadrilateral $A B C D$ is $\qquad$ .
2.1. Isosceles Trapezoid or Rectangle.
When $A D \neq B C$, by $A D / / B C$ we get $A B=C D$, at this time, quadrilateral $A B C D$ is an isosceles trapezoid.

When $A D=B C$, then quadrilateral $A B C D$ is a parallelogram. Therefore, $\angle A=\angle C$. Also, $\angle A+\angle C=180^{\circ}$, so, $\angle A=$ $90^{\circ}$. At this time, quadrilateral $A B C D$ is a rectangle.