Definition-let a, b ,c and d are the four points in a plane such that-
1. No three of them are collinear
2. The line segment ab BC CD and DA do not intersect except at their and points.
Sides-in a quadrilateral ABCD the four line segment AB, BC , CD and DA ,are called its side.
Adjecent Sides-Two sides of a quadrilateral are called its adjacent sides , if they have a common end points.
Opposite sides-Two sides of a quadrilateral are called its opposite sides if they do not have a common end point.
Diagonals- In the quadrilateral ABCD the line segment AC and BD are called its diagonal.
Angles- In the quadrilateral ABCD the angles, angle DAB , angle ABC angle BCD and angle CDA are called its angle. These angles are denoted by angle a angle b angle c and angles respectively.
Adjacent angle-two angles of a quadrilateral are called Adjacent angle, is they have a common side as an arm.
Interior and exterior of a quadrilateral.
Figure 1-shows interior and exterior angle of a quadrilateral.
Convex and concave quadrilateral
Definition-a quadrilateral is called convex quadrilateral if the line containing any side of the quadrilateral has the remaining vertices on the same side of it.
Convex and concave quadrilateral shows in figure 2
I hope this helps you!!!