The Collaborative International Dictionary of English v.0.48:

machine code \machine code\ n. (Computers)
   Same as machine language.
   [WordNet 1.5] machine-controlled

WordNet (r) 3.0 (2006):

machine code
    n 1: a set of instructions coded so that the computer can use it
         directly without further translation [syn: machine code,
         machine language]

The Free On-line Dictionary of Computing (30 December 2018):

machine code
machine language

    The representation of a computer program that is
   read and interpreted by the computer hardware (rather than by
   some other machine code program).  A program in machine code
   consists of a sequence of "instructions" (possibly
   interspersed with data).  An instruction is a binary string,
   (often written as one or more octal, decimal or
   hexadecimal numbers).  Instructions may be all the same size
   (e.g. one 32-bit word for many modern RISC
   microprocessors) or of different sizes, in which case the
   size of the instruction is determined from the first word
   (e.g. Motorola 68000) or byte (e.g. Inmos
   transputer).  The collection of all possible instructions
   for a particular computer is known as its "instruction set".

   Each instruction typically causes the Central Processing
   Unit to perform some fairly simple operation like loading a
   value from memory into a register or adding the numbers in
   two registers.  An instruction consists of an op code and
   zero or more operands.  Different processors have different
   instruction sets - the collection of possible operations
   they can perform.

   Execution of machine code may either be hard-wired into the
   central processing unit or it may be controlled by
   microcode.  The basic execution cycle consists of fetching
   the next instruction from main memory, decoding it
   (determining which action the operation code specifies and
   the location of any arguments) and executing it by opening
   various gates (e.g. to allow data to flow from main memory
   into a CPU register) and enabling functional units
   (e.g. signalling to the ALU to perform an addition).

   Humans almost never write programs directly in machine code.
   Instead, they use programming languages.  The simplest kind
   of programming language is assembly language which usually
   has a one-to-one correspondence with the resulting machine
   code instructions but allows the use of mnemonics (ASCII
   strings) for the "op codes" (the part of the instruction
   which encodes the basic type of operation to perform) and
   names for locations in the program (branch labels) and for
   variables and constants.  Other languages are either
   translated by a compiler into machine code or executed by an
   interpreter

   (2009-06-16)