Questions & Answers about Reisen er én sjømil.
Reisen is the definite form of the noun reise, so it means the journey / the trip.
- reise = a journey / a trip
- reisen = the journey / the trip
So the sentence is talking about a specific journey, not just any journey.
Also, reise is a noun that is often treated as feminine in Norwegian, but in Bokmål many feminine nouns can also use the common-gender ending -en in the definite singular. That means both of these can exist in Bokmål:
- reisen
- reisa
Both can mean the journey, though reisen is the form used here.
Én is the number one, written with an accent to make it clear that it is being used as a numeral and not as an article.
Compare:
- en sjømil = a nautical mile
- én sjømil = one nautical mile
In many cases, Norwegian can write just en, but én is often used when the speaker wants to emphasize the number or avoid ambiguity.
So here, én sjømil clearly means one nautical mile.