Midpoint snaps to the midpoint of objects such as lines or arcs.
On infinite lines and rays, Midpoint snaps to the first point defined (the root). When you select a spline or an ellipse, Midpoint snaps to a point on the object that is halfway between the starting point and the endpoint.
If you have created a line or arc with thickness, you also can snap to the midpoints of the projected edges of the object.