Magus (魔術師, Majutsushi?, plural Magi) is the term for a human practitioner of Magecraft, the magical science, as opposed to a Magician, who is capable of bringing forth true miracles that are impossible to be reenacted at the current scientific stage. In ancient times, the term magus was used for both the users of Thaumaturgy and Magic, but with the advancement of science that made past miracles possible and the decline of the overall power of Magecraft, the differences between the two became clearer. At the modern age, there are only five Magicians left. A wizard, just like a magus, has a long life. If one uses life-prolonging magic, it wouldn't be impossible to live a few hundred, a few thousand years either.
The very first magi were merely scholars that studied the supernatural, and the present level of Thaumaturgy was attained thanks to the accumulated knowledge and bloodline history started by them. Magi are frequently, but not necessarily, descend from old magical families, as affinity to sorcery is something transmitted through blood relations.[1]
More than just a name, the term "Magus" define a whole set of beliefs and lifestyle. A magus dedicates himself to the study and refinement of Thaumaturgy while driving himself apart of the values of common people in the process. He searches for the greater truths of the universe not to answer existential questions, but just for the sake of knowing. Ideally, a magus would render his own self transparent while still retaining his ego. In reality however, most magi, especially those of the Mage's Association, are often caught up on petty power struggles. On another hand, those who openly deviate from the way of magi, such as Kiritsugu Emiya and Shirou Emiya, are perceived as heretics that do not deserve to be called a magus