in-joke
inside joke
private joke

It doesn't even come up in jokes or casual conversations.

For when Bobby was bored with work, she found relief in tricks and jokes.

In other jokes however, the lack of seriousness is more subtle.

He also often gives the punchline in jokes where all three of the trio are involved.

Unlike other Soviet leaders, in jokes he is always harmless.

But most of her jokes at the beginning were 'in' jokes.

Male dancers do not exist on commercial television except in jokes.

These things never work in real life the way they do in jokes.

Since they had no common currency, the winnings were paid off in ethnic jokes.

Some of these "in" jokes travel better than others.