Hey there,
Your question states: "What is the term used when you want to focus on training one body part".
I'm actually taking (PE) right now, online of-course and I'm learning all of these questions. So, the term that many people used when describing on focusing on one body part would be called "specficity". The reason why that would the answer would be because your using specificity on working out on that body part alone, just that part only.
Hope this helps.
~Jurgen