Yes, a trapezium is indeed a quadrilateral. In geometry, a quadrilateral is defined as a polygon that has four sides, four vertices, and four angles. A trapezium, which is sometimes referred to as a trapezoid in certain regions, fits this definition perfectly as it also has four sides.
The defining characteristic of a trapezium is that at least one pair of its opposite sides is parallel. This unique property distinguishes it from other types of quadrilaterals, such as rectangles, squares, and parallelograms. While all trapeziums are quadrilaterals, not all quadrilaterals are trapeziums, as some may not have any sides that are parallel. Thus, the trapezium holds its place within the broader category of quadrilaterals, showcasing the diverse forms that can exist within this geometric classification.