In the article on the GDT the OSDev wiki describes the flag that is used as D bit for CS descriptors as follows:
Sz: Size bit. If 0 the selector defines 16 bit protected mode. If 1 it defines 32 bit protected mode. You can have both 16 bit and 32 bit selectors at once.
Another question quotes the Intel manuals: What does the D flag in the code segment descriptor do for x86-64 instructions? which links to the part "3.4.5 Segment Descriptors" from Intel 64 and IA-32 Architectures Software Developer’s Manual Volume 3 [...]: System Programming Guide, reading:
D/B (default operation size/default stack pointer size and/or upper bound) flag
Performs different functions depending on whether the segment descriptor is an executable code segment, an expand-down data segment, or a stack segment. (This flag should always be set to 1 for 32-bit code and data segments and to 0 for 16-bit code and data segments.)
• Executable code segment. The flag is called the D flag and it indicates the default length for effective addresses and operands referenced by instructions in the segment. If the flag is set, 32-bit addresses and 32-bit or 8-bit operands are assumed; if it is clear, 16-bit addresses and 16-bit or 8-bit operands are assumed. The instruction prefix 66H can be used to select an operand size other than the default, and the prefix 67H can be used select an address size other than the default.
The question is, what does "D" stand for?
I found a copy of the Intel 80386 Programmer's Reference Manual, 1987 which has the following descriptions in 16.1 How the 80386 Implements 16-Bit and 32-Bit Features:
The features of the architecture that permit the 80386 to work equally well with 32-bit and 16-bit address and operand sizes include:
The D-bit (default bit) of code-segment descriptors, which determines the default choice of operand-size and address-size for the instructions of a code segment. (In real-address mode and V86 mode, which do not use descriptors, the default is 16 bits.) A code segment whose D-bit is set is known as a USE32 segment; a code segment whose D-bit is zero is a USE16 segment. The D-bit eliminates the need to encode the operand size and address size in instructions when all instructions use operands and effective addresses of the same size.
Instruction prefixes that explicitly override the default choice of operand size and address size (available in protected mode as well as in real-address mode and V86 mode).
Separate 32-bit and 16-bit gates for intersegment control transfers (including call gates, interrupt gates, and trap gates). The operand size for the control transfer is determined by the type of gate, not by the D-bit or prefix of the transfer instruction.
Registers that can be used both for 32-bit and 16-bit operands and effective-address calculations.
The B-bit (big bit) of data-segment descriptors, which determines the size of stack pointer (32-bit ESP or 16-bit SP) used by the CPU for implicit stack references.
So "D bit" stands for "Default operand and address size" (for code segments) and "B bit" for "Big" (for stack segments).