Probably the Galactic Civil War and the Clone Wars
At the beginning of Episode IV, we see Princess Leia escaping from the forces of the Empire after the plans of the Death Star have been stolen.
Now, according to Wookieepedia refers to the 'Galactic Civil War' as having its roots back in the Clone Wars, but that:
The Alliance scored its first major victory against the Empire when it
stole the plans to the Death Star, the Empire's planet-destroying
That is the beginning of Episode IV!
When we consider that over the two trilogies, there are two central 'wars'; the Clone Wars and the Galactic Civil War, it seems most likely to me that the title is referring to these two wars, which form the basis of the plots throughout the trilogies.
Think about it:
- Episode I: we have the invasion of Naboo, leading to tensions between The Republic and Separatists escalating, leading to the Clone Wars
- Episode II: we have the beginning of the Clone Wars
- Episode III: we have the end of the Clone Wars
- Episode IV: essentially the beginning of the Galactic Civil War; the Rebels' first major strike against the Empire
- Episode V: the Empire strikes back against the Rebels, continuing the Galactic Civil War
- Episode VI: the Empire and Rebels continue fighting in the Galactic Civil War
Despite all the battles, we really only have the two wars throughout the trilogies - one per trilogy. Hence, the title of 'Star Wars' is probably referring to these two wars. Bear in mind that Lucas had prepared the ideas for the first three films before making the original trilogy, so he probably had this intention in mind.
Also note that I'm only focusing on the core canon materials - the movies; I'm not even scratching the surface of the rest of Legends material!
From a purely out-of-universe perspective, it's interesting to note that:
Lucas wrote a short summary called "The Journal of the Whills", which
told the tale of the training of apprentice CJ Thorpe as a
"Jedi-Bendu" space commando by the legendary Mace Windy.
Frustrated that his story was too difficult to understand, Lucas then
began writing a 13-page treatment called The Star Wars on April 17,
1973, which had thematic parallels with Akira Kurosawa's The Hidden
After revisions, Lucas wrote more drafts (my emphasis):
The film was titled Adventures of Luke Starkiller, as taken from the
Journal of the Whills, Saga I: The Star Wars. During production, Lucas
changed Luke's name to Skywalker and altered the title to simply The
Star Wars and finally Star Wars.
So, one aspect about the title is its simplicity - Star Wars is certainly more catchy than Adventures of Luke Starkiller, as taken from the Journal of the Whills, Saga I: The Star Wars!