One who assembles a number of individuals; also, one of a number assembled. [1913 Webster]
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]
assembler, n.
1 a person who assembles a machine or its parts.
2 Computing a a program for converting instructions written in low-level symbolic code into machine code. b the low-level symbolic code itself; an assembly language.