What is the difference between a microoperation, microinstruction and control word?
In traditional horizontal microcode,


a micro-operation is the most basic operation achievable by a processor;
a micro-instruction is a set of micro-operations which are executable simultaneously;
control words are the bit patterns used to encode the micro-instructions.




Micro-operation is also used in current pipelined processors where an instruction is decomposed in a few (included zero) micro-operations or even a few instructions merged in one micro-operation which are then processed independently.

Control word is a far more general term and can refer to any bit patterns used to control any circuit.  Its use for the encoding of micro-instructions come from the fact that horizontal micro-code is usually lightly encoded if at all, and thus fields in the encoding control directly parts of the processor.  From there it went to be used for the encoding of any kind of micro-instructions.  BTW, the area where the micro-program is stored is often called the control store.