Overconcentrated shape is a translation of the Japanese term korigatachi[1]. It means too many stones (of one colour) in one part of the board for the work they do; so shape that is inefficient because too solid.
Dosaku, the undisputed best player of his time, brought go to a new level of theoretical knowledge. It was he who introduced the concept of tewari analysis. Applying this, he tried to overconcentrate the shapes of his opponents, like in this 1670 game against Chitetsu.
After this sequence the black formation at the top is very strong. However the influence of the black groups overlaps in the marked area. This means that these stones have not been developed to their full efficiency on the board: they are overconcentrated.
--Stefan
Charles By Shusaku's day, I think all pros knew about avoiding overconcentration traps.
[1]
Often it is stated that this is literally frozen shape.
Fhayashi:That would be ko-o-ri gatachi, while I've seen it written in the Fujisawa Tesuji Dictionary as ko-ri gatachi, in katagana. I suspect the kori refers to 'cramped', which seems to fit the usage (in the aforementioned dictionary, at least).
John F. Fhayashi is correct, Koru is used of congealing, clotting, gettting stiff shoulders, etc. Kooru (freeze) is a separate though probably cognate verb.
Bill: Shall we change it to cramped shape?