As other pointed out, there's nothing in C standard(s) such as .NET's decimal, but, if you're working on Windows and have the Windows SDK, it's defined:

DECIMAL structure (wtypes.h)

Represents a decimal data type that provides a sign and scale for a number (as in coordinates.)

Decimal variables are stored as 96-bit (12-byte) unsigned integers scaled by a variable power of 10. The power of 10 scaling factor specifies the number of digits to the right of the decimal point, and ranges from 0 to 28.

typedef struct tagDEC {
  USHORT wReserved;
  union {
    struct {
      BYTE scale;
      BYTE sign;
    } DUMMYSTRUCTNAME;
    USHORT signscale;
  } DUMMYUNIONNAME;
  ULONG  Hi32;
  union {
    struct {
      ULONG Lo32;
      ULONG Mid32;
    } DUMMYSTRUCTNAME2;
    ULONGLONG Lo64;
  } DUMMYUNIONNAME2;
} DECIMAL;

DECIMAL is used to represent an exact numeric value with a fixed precision and fixed scale.

The origin of this type is Windows' COM/OLE automation (introduced for VB/VBA/Macros, etc. so, it predates .NET, which has very good COM automation support), documented here officially: [MS-OAUT]: OLE Automation Protocol, 2.2.26 DECIMAL

It's also one of the VARIANT type (VT_DECIMAL). In x86 architecture, it's size fits right in the VARIANT (16 bytes).