It was named after 16th century Czech naturalist Tadeáš Hájek (Latinized as Thaddaeus Hagecius).

Like both of these craters, Hagecius has undergone erosion from subsequent impacts, and its outer rim is worn and irregular.

About a third of the interior floor on this side is also overlaid by the outer ramparts of the three satellite craters mentioned above.

To the northeast of Hagecius is the crater Biela, a younger formation with more distinctive features.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Hagecius.