The Vision is always described as a Synthezoid, however I can't find any description of what that is other than a "A special type of android empowered by synthesizing energy". Is there any real difference between the two, or is it just semantics?
It is mostly a matter of semantics. The Vision's only real difference than any other android in the Marvel universe is the synthetic matter which comprises his body and replicates human organs as well as providing him with the dazzling array of abilities he is capable of performing. There is, however, another android who could also lay claim the title of synthezoid as well. That would be the original Human Torch created by Professor Phineas Horton. Technically both superheroes are using the same synthezoid body.
The Vision possesses a number of superhuman powers ultimately derived from his artificial substance and metabolism. The Vision's android body is functioning replica of a human body containing analogues to virtually all human organs, blood, and tissue, composed of an unrevealed synthetic organic-like substance. This substance mimics all the functions of human tissue, but is several times as strong, durable and resilient.
He has been killed and rebuilt several times over his career, so in that respect he is similar to any other android albeit comprised of a very advanced base material which after all these years has still not been patented or replicated by Ultron, Hank Pym or Iron Man. The Vision has been rebuilt by Iron Man, Hank Pym and has been repaired by Reed Richards and Ultron.
The Vision being analysed before his defeat at the hands of a Sentinel. Note the reference to his "vintage" design and solar technology update.
Ultron did not invent the initial technology used to create the underlying body of the Vision. The actual creator was Professor Phineas Horton and the original body was the World War II superhero, the Human Torch who had a brief career as a superhero with the Invaders.
In an event with the time traveling conqueror, Immortus, the android Human Torch was temporally duplicated and one of those duplicates fell into the hands of the Mad Thinker and the other into the hands of the killer robot, Ultron to create his weapon, the Vision.
Yes, there are still two modern superheroes who call themselves the Human Torch. One was rebuilt by the Mad Thinker and returned to active service. The other is the team member of the Fantastic Four whose powers were caused by an interaction with mysterious "cosmic rays" during a space flight in an improperly shielded ship.
The history of the Vision has been retconned, rewritten, and hand-waved for over 40 years. I suspect when Marvel created the Vision from the body of the android Human Torch, at the time, no one was planning to do anything with the original Human Torch. But when they decided to open the can of worms by bringing the original Human Torch back to life, they had some 'splaining to do.
It was never explained where the technology to create the original Human Torch came from. No one ever questioned how the Horton android was so advanced considering the technology of the time. The original Human Torch would have been far more advanced than any technology from Earth at the time. And in the case of the Vision would still remain so, 40 years later.
Immortus was introduced to have a convenient rational for there being two beings who could claim to be made from the P. Horton android body and to cover for what would have become a continuity error if left uncorrected.
It was assumed the technology used to create the Vision was all Ultron's development, but the only thing Ultron can take credit for is the suite of powers the Vision had. He may have added other technology but the underlying energy management hardware was already there.
The knowledge and technology used to create the android Human Torch was supposedly created in the 1940s and yet has never been duplicated, why? The most scientifically advanced creator of androids/robotic intelligence on the Avengers is Hank Pym, yet his most advanced robotic creation was Ultron and to a lesser extent Jocasta. Pym is at least 40 years later than Horton, yet no androids have been created by anyone as sophisticated as the Human Torch or the Vision.
I think the Vision addressed this in the Vision and Scarlet Witch limited series. An android is created to mimic the look and behavior of a human being. The Vision is a synthetic body that housed a duplicate of the brain patterns of a human being (Simon Williams). So, a synthezoid is a human being with an artificial body.
The Vision was created by bonding vibranium with human tissue, and the A.I. Jarvis, was uploaded to the brain of The Vision, so in essence, he is an extremely advanced human body with A.I. intelligence.