Definition of "in-joke"

in-joke

/ˈɪnˌd͡ʒəʊk/

noun
plural in-jokes
1. Synonym of inside joke (“a joke that is understood or meant to be understood only by certain people who are aware of the details”)
  Quotations
  Perhaps these quotations arose from some in-joke of Yule’s, but whatever the case the lexicographical worth of these citations is dubious.
  2018, James Lambert, “Setting the Record Straight: An In-depth Examination of Hobson-Jobson”, in International Journal of Lexicography, volume 31, number 4, page 494
  Intentionally or not, the movie [Ralph Breaks the Internet] makes Disney feel as enormous as the internet itself, containing a series of micro-targeted idiosyncrasies and in-jokes that are nonetheless controlled by a cultural monolith (whether that's Disney or whatever massive corporation owns your local ISP).
  2018 November 14, Jesse Hassenger, “Disney Goes Viral with an Ambitious, Overstuffed Wreck-It Ralph Sequel”, in The A.V. Club‎, archived from the original on 21 November 2019