This is the 402nd most frequent Russian word.
нижегородской
Relating to Nizhny Novgorod or the Nizhny Novgorod region in Russia.
In this sentence, 'Нижегородской' is in its genitive form to modify the noun 'области' (region), indicating possession or association.
Я посетил Нижегородской области деревню.
I visited a village in the Nizhny Novgorod region.
Here, 'Нижегородской' is in the prepositional case to agree with 'мэрии' (city hall), showing location.
Архитектура в Нижегородской мэрии красива.
The architecture in the Nizhny Novgorod city hall is beautiful.
'Нижегородской' appears in its instrumental case here, describing 'улице' (street), indicating the means or media.