The Free On-line Dictionary of Computing (18 March 2015):
(Or "assembly code") A symbolic representation of
the machine language of a specific processor. Assembly
language is converted to machine code by an assembler.
Usually, each line of assembly code produces one machine
instruction, though the use of macros is common.
Programming in assembly language is slow and error-prone but
is the only way to squeeze every last bit of performance out
of the hardware.
Filename extension: .s (Unix), .asm (CP/M and others).
See also second generation language.