Interior of a triangle is an area which lies within the boundaries of a triangle.
Or In order words,
Interior of a triangle is an area which lies within the three sides of a triangle.
Or we can also say that;
An area within the boundaries or perimeter of a triangle is known as interior of a triangle.
Example 1: In the below Triangle ABC:
Area highlighted in green is referred to as Interior of triangle ABC.
Example 2: In the below Triangle ABC:
Points x, y & z lies in the interior of triangle ABC.

