This is the 268th most frequent Norwegian word.
sykehus
Hospital.
Here, 'sykehuset' is the definite form of 'sykehus', used to specify that it is the particular hospital being referred to in this context.
Jeg jobber på sykehuset.
I work at the hospital.
In this sentence, 'sykehus' is in its indefinite form, referring to 'a hospital' without specifying which one.
Vi besøkte et sykehus i Oslo.
We visited a hospital in Oslo.
'Sykehusene' is the plural definite form of 'sykehus', used here to refer to all hospitals in Norway inclusively.