My understanding -
In Arduino land, a shield is a daughter board with pinouts that are designed to sandwich on a standard Arduino board. It attaches like a 'shield' to a known Arduino form-factor. Daughter board is a more generic term, and might refer to any board that attaches to larger board. The sensor board on the APM2 is a good example of an arduino-related daughter board that is not a shield. Technically, one might make the distinction that the oilpan is not an Arduino shield, but it could be called an APM shield, perhaps, if there were other boards designed to be attached in place of the oilpan. You could ask the Arduino team where the term came from, and for a more authoritative answer.
It is my understanding that "oilpan" was a nickname given to the APM sensor board. Again, you might get a more authoritative answer here, and might get details on who named it, and why they chose "oilpan," but it's just a nick name, as far as I know. Easier to call that than "IMU, I/O, and sensor board for APM"
"Arduino" is like a religion for most of the people involved with it. So of course they have to make up silly names for things that already have names.
One guy put it thus, "Arduino is like the Religion of microcontrollers; everyone knows they're junk, but they show up on Sunday because that's where their friends are."
It would sound pretty silly if us Christians talked about believing in ghosts, magic, and all prayed to the "Holy Ghost", so instead they now use the terms angels/souls, miracles, and God.
These words originate from engineers and devs who probably need to get more sun ;-) They are just an easy and cute way of quickly referring to what are technical and usually difficult to describe easily in normal conversation.
Unfortunately you have to study up and be 'in the know' as a lot of these terms are peculiar to this project.