Any plane polygon having four or more sides may have one pair of parallel sides.
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A trapezoid is a quadrilateral with two parallel sides.
A quadrilateral with one pair of parallel sides is called a trapezium (most of the world) or trapezoid (US) . If it was to have two pairs of parallel sides it would be a parallelogram.
trapezoid
For the same reason that a cat is an animal but an animal is not always a cat. Because a trapezoid is a special kind of quadrilateral. A quadrilateral is anything with four sides. A trapezoid has four sides, and also two of them are parallel to each other. Therefore some quadrilaterals are trapezoids, but most of them aren't.
No. A trapezoid has two parallel sides. This is not possible in a concave quadrilateral.