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.

Because the sentence says one nautical mile, Norwegian uses the singular form:

• én sjømil = one nautical mile
• to sjømil or more commonly to nautiske mil in some contexts = two nautical miles

After one, you normally use the singular noun, just as in English:

• one mile
• one nautical mile

So sjømil is singular here.

Sjømil means nautical mile.

It is a unit used especially at sea and in navigation. One nautical mile is:

• 1,852 meters
• about 1.852 kilometers

So this is not the same as an ordinary land mile.

Er is the present tense of å være, which means to be.

So:

• er = is / are

In this sentence, Reisen er én sjømil literally means The journey is one nautical mile.

This is the normal Norwegian way to link the subject (reisen) with the measurement (én sjømil).

It is understandable, but many learners will notice that Norwegian often prefers to add lang in this kind of sentence:

• Reisen er én sjømil lang.

That means The journey is one nautical mile long.

Without lang, the sentence can still be understood, especially from context, but ... lang can sound more explicit and natural when talking about length or distance.

It overlaps with all of those, depending on context.

Reise can mean:

• trip
• journey
• sometimes something close to travel

In this sentence, journey or trip is the most natural English translation.

The exact best English word depends on context, but grammatically the Norwegian noun is straightforward here.

Yes, in Bokmål, many feminine nouns have two possible definite singular forms.

For reise, you may see:

• reisen
• reisa

Both can mean the journey.

This happens because Bokmål often allows either:

• a common gender pattern
• or a feminine pattern

So a learner should recognize both forms, even if one textbook or speaker prefers one over the other.

The word order is very simple and follows the basic Norwegian pattern:

• Reisen = subject
• er = verb
• én sjømil = complement

So it is:

Subject + verb + complement

This is the same basic order as in English:

• The journey
  • is
    • one nautical mile

That makes this sentence fairly easy for an English speaker to understand structurally.

The beginning sj- in Norwegian is usually pronounced like an English sh sound.

So sjø begins roughly like sh + a vowel sound that English does not have exactly.

A rough approximation for English speakers is something like:

• sjø ≈ shur/shuh with rounded lips

You do not need a perfect accent at first. The important part is that sj is not pronounced like English s or j separately. It acts as one sound, similar to sh.