definition
strict definition
Wikidata item ID
The identifier of the matching item in Wikidata, the free knowledge base run by the Wikimedia Foundation, written as a Q followed by a number.
A Wikidata item ID is the key SKYDB uses to line a record up with the same subject elsewhere. Q243 is the Eiffel Tower in every language, which is what makes the number usable where a name is not.
One item can legitimately sit on two SKYDB rows. Singapore and Monaco are a country and a city, Berlin and Hamburg a state and a city, and each pair is one Wikidata item carrying both roles.
The reverse also happens: a capital inside its country is its own item, so Luxembourg City is Q1842 and not Q32. A shared name is never proof that two records mean the same thing. See identifier for the general term.