That's D.C., as in District of Columbia. In 2010, the D.C. Circuit posted this notice:
The D.C. Circuit Rules permit the use of abbreviations, including acronyms, in briefs, provided the brief contains a glossary defining the abbreviations, other than those that are part of common usage. D.C. Cir. Rule 28(a)(3). To enhance the clarity of the brief, the court strongly urges parties to limit the use of acronyms. While acronyms may be used for entities and statutes with widely recognized initials, such as FERC and FOIA, parties should avoid using acronyms that are not widely known.
The BLT's story "In D.C. Circuit, Judge Calls Use of Acronyms 'Painful'," makes clear that at least one judge, Laurence Silberman, takes notice very seriously. Acronyze at your own risk, FWIW.