What is the proper name or part number of a 12V DC female car adapter with leads?
In 1921, the Morris U.S. Patent 1,376,154 was issued for a so-called "wireless" or "cordless" lighter. This lighter eliminated the cables and the mechanism for winding and unwinding them. The igniter was heated in the socket then removed for use.

Twelve volt automobile plugs are standardized in the United States by UL Standard 2089 regarding vehicle battery adapters. This standard covers plugs and cord sets that insert into cigarette lighter receptacles.

The sockets and mating plugs are defined in the ANSI/SAE J563 specification

Those three are listed in wikipedia. Turns out that these don't really have a fixed name past lighter receptacle/socket/assembly/etc. Anyone knows what you mean by just calling it a cigarette lighter assembly. Even the above standards don't give them a special name, but you have to purchase the standard to read the entire thing so they might have a name buried in them.

Digikey has them as "cigarette lighter assemblies" under the batteries category (doesn't make sense I know) http://www.digikey.com/catalog/en/search?filters=18648