Assembly language
An assembly (or assembler) language, often abbreviated asm, is a low-level programming language for a computer, or other programmable device, in which there is a very strong (but often not one-to-one) correspondence between the language and the architecture's machine code instructions.
Each assembly language is specific to a particular computer architecture. In contrast, most high-level programming languages are generally portable across multiple architectures but require interpreting or compiling.
Assembly language may also be called symbolic machine code.
Explanation
In order to work with assembly the concept of machine code and the ISA must be understood.
On this low abstraction level (no matter if reversing or engineering), assembly is tied to the the particular instruction set.
Which in most cases is a variant of x86 / x86-64 or ARM.
I found this example very helpful in order to understand how to the translation between mnemonic and opcode works.