This is the 6088th most frequent Polish word.
amerykę
"Amerykę" is the accusative form of "Ameryka," meaning "America" in English.
In this sentence, 'Amerykę' is used in its direct object form, the accusative case, as the action of visiting requires this case for the noun.
Odwiedziłem kiedyś Amerykę.
I once visited America.
Here, 'Ameryki' is in the genitive case, which is required after the preposition 'o' when expressing a dream or desire about something.
Marzę o podróży do Ameryki.
I dream of traveling to America.
In this context, 'Ameryki' is used in the genitive case required by the preposition 'do' indicating movement toward a place.