Roddenberry's memo to Jon Povill vis-á-vis forcefield terminology, Oct. 13, 1977 (from the Reeves-Stevens book ST Phase II - the Lost Series
, p.50), contained these pieces of wisdom:
"FORCEFIELD SCREEN - a defensive or protective field which envelops or surrounds a vessel, person or object. A FS can also be called simply a 'forcefield', still means the same thing. We never use the term 'forcefield shield' as 'shield' refers to a single plane of forcefield protection as opposed to protection which envelops or surrounds [the target]." The memo then mentions the automated operation of the forcefields, the ability to drop sections of the forcefields, the ability to use communicators or transporters through weak forcefields, and the idea that the helmsman controls the forcefields.
"DEFLECTORS or DEFLECTOR SHIELDS - another kind of vessel protection which are the responsibility of the weapons and armament officer. Unlike a forcefield, a deflector shield does not surround the vessel. It is a single plane of protection as the term shield would indicate. Deflector shields can be many, and thrown up in manuy difrections, and they generally supplement the forcefield shields." The idea is that the strong shields can only be held up against attacks from so many directions, and a clever and determined enemy can get around them eventually.
"DEFLECTOR BEAM - this is solely a navigational device. It is never referred to simply as a 'deflector'." It's then described as in the TNG Tech Manual, as a forward-pointing "sweeper" beam for clearing the flightpath.
The terminology in that memo is somewhat confusing and contradictory, even though the ideas on the various sorts of protective fields are nicely developed. Whether the above was still in vogue when TMP dialogue was written is debatable, but it seems to fit the onscreen material pretty well.