I don't know what this is called - Negative part or part holder

[coyotebgone]

Attached,  I slapped together some shapes and am calling them my final part.  

My goal would be to produce the mirror part as a negative, example for parts holding or mold making.  Can you tell me how to do this. 

I have tried offset planes and midplanes.  Each time, when i extrude the plane, it absorbs the original part as a part.  

This is for CNC Milling, if that matters. 

Thank you. 

[HughesTooling]

The combine tool set to cut should do what you're after. Also have a look through this thread on Rule #1, using components will make it a lot easier. Screencast below and the file's attached.

Mark

Edit forgot the link to the Rule #1 thread.