To inscribe a polygon (such as a triangle, square, pentagon, etc.) or circle is to enclose it in another polygon or circle so that the vertices of the polygon are on the circumference of the circle or sides of the outer polygon, or so that the circumference of the inner circle touches the sides of the enclosing polygon. In the specific case of a polygon inside of a circle, this is called to circumscribe.

Now, a couple of examples: First, a circle inscribed in a pentagon.

Next, another example of a pentagon, but inscribed with a triangle this time.