One thing is how will we define terrorism? On a basic level it's to cause terror, obviously. Being bombed will certainly do that. The question also is whether or not you accept that terrorism can be committed by a military force which is prosecuting a war. The Online Etymology Dictionary says that the term "terror bombing" was first recorded in 1941 in regards to the German bombing of Rotterdam. Since WW2 a number of people, including historians, have used Terror Bombing to describe the various Allied strategic bombing campaigns (the ones where cities were bombed, sometimes pretty flat).

Another question is was the bombing of Japan illegal? This is important depending on how you define terrorism. If you don't consider the legal prosecution of warfare to be terrorism then you may consider the illegal prosecution of warfare to be terrorism. The Hague Convention prohibited the bombardment of undefended cities. The Japanese certainly defended their cities and territory. For instance they had anti aircraft guns. They also had an airforce, even though this dwindled as the war went on. Also, there were no laws/rules in the Hague Convention which made aerial bombardment in particular illegal due to the fact that the conventions were from 1899 and 1907 which is before aircraft and then only just after. Their full potential for warfare had not yet being realised.

This of course brings up the issue to the atomic bombings, which were mentioned in the link. Are they any different to conventional bombs? This of course has been greatly argued since the end of WW2 and could be an entire other topic (and has been, many times).

So, at the end of the day, Would (I'd use could) the American bombing of Japan during WWII be considered terrorism? Possibly, depends how you define it.

Edit - I'm about to rush off to work so I haven't had time to find as much as I would have wanted. One term you may want to look up is "Dehousing" which is a term the Allies used during WW2 where they pretended that they were destroying the German's houses and not killing the occupants. This was on the basis that people really don't like their homes being destroyed and would hopefully break their morale. So, on the same as the above, could that be considered terrorism and then if so does that help define the bombing of Japan as terrorism?