For example, I have heard from various sources the difference between a "Battlefield weapon" like a Longsword as opposed to a "sidearm" sword, but was the specific term "Sword of War" or a direct translation thereof, a term used by people in the late medieval to early modern ages?
There are two main types of European sword the names of which can be translated into English as "sword of war". The earlier of the two types is from the 13th and 14th centuries, and "sword of war" is a direct translation of contemporary names like "epee de guerre". These swords typically have blades of about 90-100cm, and hilts of about 25cm. They are smaller, and much lighter, than the big Renaissance two-handed swords. Still, they are larger, and often heavier, than their contemporary one-handed brethren, and were also called "great swords" as well as "swords of war". Their name might result from their size: less convenient for regular wear as a sidearm, making them more of a weapon one only took to war.
An example of such an epee de guerre:
For the other type, the big Renaissance two-handed sword mentioned just above, "sword of war" is perhaps a less accurate translation. A contemporary German name for this type was "Schlachtschwert", literally "battle sword". The German name was adopted in English, anglicised to "slaughter sword".
Some examples: