This is the 511th most frequent Norwegian word.
hemmelig
Secret.
Here, "hemmelighet" is the noun form of "hemmelig" and means "secret."
Jeg fortalte henne en hemmelighet.
I told her a secret.
In this sentence, "hemmelig" is used as an adjective to describe the noun "møtet" and means "secret."
Møtet er hemmelig.
The meeting is secret.
Here, "hemmelig" is used adverbially to mean "secretly."