Learn Icelandic: Meðaltalið er tíu þúsund.

It actually is plural! In Icelandic, þúsund behaves like a neuter noun. Just like the neuter word hús (house) stays hús in the plural, þúsund stays exactly the same in the nominative plural. So, eitt þúsund (one thousand) and tíu þúsund (ten thousand) use the exact same form.

The word meðaltalið looks like it's made of two parts. What does it literally mean?

You're right! It's a compound word made from meðal, which means "among" or "mean", and talið, which is the definite form of tal ("number" or "count"). Put them together, and you literally get "the mean number," which is how Icelandic expresses "the average".

I remember that numbers 1 through 4 change based on gender. Do I need to worry about gender when using tíu?

Thankfully, no! In Icelandic, only the numbers 1 through 4 change their endings to match the gender and case of the noun they are counting. Numbers from 5 onwards, including tíu (ten), do not change for gender.

Why is the number tíu þúsund in the nominative case here?

It's because of the verb er (is). The verb vera (to be) acts like an equals sign in a sentence rather than an action. It links the subject (meðaltalið) to an equivalent value. Because of this, both sides of the "equals sign" stay in the nominative case.