This is the 551st most frequent Finnish word.
viikkoa
"Viikkoa" is the partitive singular form of "viikko," meaning "week" in English.
The word 'viikkoa' here is in the partitive case, indicating a duration of time.
Olen ollut lomalla kaksi viikkoa.
I have been on vacation for two weeks.
The word 'viikon' here is in the genitive case, indicating a time reference.
Näemme taas viikon päästä.
We will see each other again in a week.
The word 'viikko' here is in the nominative form, acting as the subject of the sentence.