Adjacent sides

Two sides of any polygon that have a common vertex are called adjacent sides.

In the triangle ABC sides AB and AC are adjacent sides as they meet a common vertex that is A.

In the angle ABC sides AB and BC are adjacent sides as they meet a common vertex that is B.

In the rectangle ABCD sides AB and BC are adjacent sides as they meet a common vertex that is B, 

sides BC and DC are adjacent sides as they meet a common vertex that is C, 

sides AD and DC are adjacent sides as they meet a common vertex that is D, 

sides AB and AD are adjacent sides as they meet a common vertex that is A.