Here are the exact definitions as described on the Glossary of Structural Kinesiology & Weight Training.
A principal exercise that can place greater absolute intensity on the muscles exercised relative to auxiliary exercises.
An optional exercise that may supplement a basic exercise. Auxiliary exercises may place greater relative intensity on a specific muscle or a head of a muscle.
An exercise that involves two or more joint movements.
An exercise that involves just one discernible joint movement.
Diet is the most important piece of the equation to achieve any fitness goal.
When performing exercises compound & functional movements are more important than isolated abdominal movements for they will use more of the core to stabilize the body as it naturally does & needs.
Try getting off the bench and use dumbbells on the Stability Ball. It will not impede your scapula. Compound lifts do utilize the core for stability and support. I believe the abdominals are firing more in this environment than that of isolation of them alone.
Personally I never train "bodyparts", but instead I train movements. I do my best to synergistically work the body as nature intended by using compound movements most of the time. I rarely- almost never- isolate.
My intensity is always high and would never go into the gym to waste my time with a light day.
In training I have adapted the Bruce Lee methods: Use no way as way and no limitation as limitation. Anything goes as long as it works. Throw away what does not- keeping only that which does.
When training always remember: Less Is More!