Definitions are hard to garner when it comes to seacocks. Here is my best interpretation of it from ABYC.
Proper Seacock= A tapered cone or ball valve type valve with a handle that rotates only 90 degrees that also has an integral flange. Whether this is on or off the boat it is still called a seacock as that is its only designed purpose.
Ball Valve = This is still called a ball valve until you install it on the top of a flanged adapter or a thru-hull fitting then it becomes a seacock. While it is not a seacock in the "proper" sense any valve that penetrates the hull becomes a seacock if it meets the 90 degree handle rotation definition set forth in ABYC H-27.
Ballcock = This is a plumbing term used by the plumbing trade and only defines the style of valve as a "ball" type "****". The words "****" or "valve" are indeed interchangeable. However there are many types of cocks such as sillcocks, which can also be of the ball type, and petcocks, which can also be of the ball type. Ballcock is not a term generally used on boats but is not necessarily wrong unless you are talking to someone in the marine trade who is expecting to hear the term seacock and you are saying ballcock which could indicate a valve not attached to the hull.
As for your other question on another post:
From ABYC H-27
"27.2.2 Threads used in seacock installations must be compatible. (e.g. NPT to NPT, NPS to NPS)"
I think this is pretty clear wording and clarifies that many of the installations currently out there, including your engine intake, do not meet the current ABYC safety standard for a seacock installation.
Properly flanged seacocks or the use of flanged adapters or matched threads and the ability to withstand a 500 pound static load at the most inboard portion of the assembly (usually the hose barb) are the only installations that will meet the current ABYC H-27 safety standards. If you feel comfortable that the mis-matched threads and valve could hold a 500 pound static load at the innermost hard fitting on the valve assembly then you're good to go..
-Maine Sail / CS-36T
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