Brass
Brass was a metal alloy on Earth, composed of copper and zinc.  The alloy was used as a material in various pieces of equipment and structures as late as the 24th century. (; )  

In terms of military parlance, by the late 23rd century the term "top brass" was a phrase used to describe Starfleet Command, specifically the many admirals who were adorned with various pins and badges on their Starfleet uniform.  A high level decision, involving several admirals, was said to be one which was "up to its brass". ()

 

Pavel Chekov, attending one such Starfleet Command briefing in 2293, commented that he expected the meeting was only intended for "top brass" and was confused why the crew of the USS Enterprise had been invited. ()

Another colloquial saying using the word "brass", was "brass hat". (; )

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Memory Alpha figure of speech redirects