Define opposite sides of a quadrilateral.

Hint: Draw a quadrilateral, it has 4 sides, if the 4 sides are equal then it is called a square, if the pairs of sides are equal, it is called rectangle there can be a thousands of different shapes that can be formed using these 4 sides and still they all will be termed as quadrilaterals.

Complete step-by-step answer:
A simple closed figure formed by joining four line segments is called a quadrilateral.
It has 4 sides, 4 angles, 4 vertices and two diagonals.
In a quadrilateral if A, B, C, D are four points in a plane such that no three of them are collinear and the line segments AB, BC, CD and DA do not intersect except at their endpoints.
Then, the figure formed by these four line segments is called the quadrilateral ABCD.
In a quadrilateral ABCD


(i) the four points A, B, C, D are called its vertices,

(ii) the four line segments AB, BC, CD and DA are called its sides,

(iii) \[\angle DAB,\angle ABC,\angle BCD\& \angle CDA\] are called its angles, to be denoted by \[\angle A,\angle B,\angle C\ & \angle D\]respectively, and

(iv) the line segments AC and BD are called its diagonals.

Two sides of a quadrilateral are called its opposite sides if they do not have a common endpoint.
In the given figure, (AB, DC) and (AD, BC) are two pairs of opposite sides of quadrilateral ABCD.

Note: The given figure does have 4 sides AB,CD,BC,DA but the figure is not closed by lines. Thus it is not a quadrilateral, as a quadrilateral must be a closed figure with 4 sides.