This is the 10721st most frequent Russian word.
восточной
"Восточной" means "eastern" in English.
The word "восточной" is used here as an adjective that modifies "part", describing its location relative to the city — eastern.
Он живёт в восточной части города.
He lives in the eastern part of the city.
In this sentence, "восточной" describes the noun "direction", specifying its orientation — eastern.
Они отправились в восточной направлении.
They left in an eastern direction.
Here, "восточной" modifies "Europe", signifying the region of Europe referred to — Eastern Europe.