The Collaborative International Dictionary of English v.0.48:

Assembler \As*sem"bler\, n.
   One who assembles a number of individuals; also, one of a
   number assembled.
   [1913 Webster]

   2. (Computers) a computer program that takes as input a set
      of instructions written in assembly language, and produces
      a corresponding executable computer program in machine
      language.
      [PJC]

   3. (Computers) same as assembly language. [informal]
      [PJC]

The Collaborative International Dictionary of English v.0.48:

assembly language \assembly language\ n.
   1. (Computers) an artificial computer language with mnemonic
      codes representing the basic machine-language instructions
      of a computer, which can be interpreted by an assembler to
      produce a computer program in machine language. Also
      informally referred to as assembler.

   Note: Writing a program in assembly language is very much
         simpler than writing the machine instructions in binary
         code, and the use of such a language greatly increases
         the efficiency of the process of writing computer
         programs. See also programming language, FORTRAN,
         BASIC.
         [PJC]

WordNet (r) 3.0 (2006):

assembler
    n 1: a program to convert assembly language into machine
         language [syn: assembler, assembly program]

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

assembler

    A program which converts assembly language
   into machine code.

   (1996-03-25)