`  : Some in-universe slang may be appropriate if it is significant to the story, explained in the first occurrence in any article that uses it, and includes a citation from a reliable source. ``Browncoat`` is a good example of a significant slang term for Firefly. But ``ident card`` isn't especially significant to the overall story. (I can think offhand of 2, maybe 3 instances where ident cards were even mentioned, and only as minor elements. The hint about Book is independent of the term itself; i.e., it doesn't matter what they called the cards, unlike the term ``Browncoat``, which impacts story elements.)  : On the other hand, ``ident card`` isn't too hard to deduce from context for native-English speakers. For non-native speakers and other readers who may not make the connection, we can include a link to Identity document, which defines what's meant and whose lead section even includes the term ``identity card``, an appropriate expansion. If we choose to use ``ident card`` with a link, we should still put ``ident`` in quotes because it is not a common usage, at least according to Cambridge Dictionaries Online and Merriam-Webster Online (although the latter apparently includes something on this term in its for-fee service). Wiktionary has an entry, but that appears to be due to our technology-heavy contributors' awareness of the ident protocol, i.e., a term not in common use outside of computer networking. ~   `