A metric unit of length equal to 1000 metres is referred to as :

1 kilometre = 1000 metre
1 mile = 1609.34 metre
1 centimetre = $\frac{1}{10}$metre

1 milimetre = $\frac{1}{1000}$metre

Therefore, we can say that a metric unit of length equal to 1000 metres is referred to as 1 kilometre.