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From The Free On-line Dictionary of Computing (05 January 2017) [foldoc]:
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)