Cultivation

The Hermit of the Ridge has been contemplating the stars for decades now. I think he has become enlightened. - Honda Sanjiro of Taan-no-Kuni

Mortals are born with certain simple limits.

They can only generate and contain so much spiritual energy. They must learn spells or produce sorcerous implements in order to harness magic. They struggle to commune with the Divine Realm. And all mortals must age and die. But limits can be circumvented.
Where sorcerers create physical constructs and modify the flesh, cultivators train internal energies and magically modify the spirit. Through meditations, asceticism, and esoteric practices, cultivators can transform themselves in ways that transcend the material world. Cultivation is one of the most difficult paths to power. Asceticism requires dedication. It is slow and often painful. Rare herbs, potions and pills can help it along, but are often expensive and poisonous. A powerful being, like a god or immortal, or a group of beings working in concert can devote their energy to guiding a mortal's spiritual advancement. This is called investiture, assisted cultivation. Many cultivators must start in childhood and dedicate years to training and nothing else. Sorcerers use trophies taken from incarnations, and witches seek to emulate their behavior. This gets them power, much sooner. Cultivators actually make themselves into beings that are partially - or completely - incarnations. In this way they cease to be fully mortal.
The most common use of cultivation is to increase the amount and type of aether the body can use. The aether of a living creature is called qi or ki. Most creatures can tap into this power to fuel the body, but it is limited. An experienced mage can put more energy into a spell due to the expansion of spirit that comes from use, the same way athletes tire more slowly. Many spellcasters meditate or perform rituals to cultivate their internal wellspring. Those with the dedication to truly focus on this practice often realize such benefits as an extended lifespan, a greater spiritual awareness, and in extreme cases, transcendence.
There is a way to use that energy directly. Instead of feeding it into a spell, some cultivators use their increased spiritual power to perform feats of physical prowess. The combination of athleticism and asceticism this requires makes it uncommon in ordinary soldiers and knights, but warrior monks are quite well known for it. Martial artists call this "internal power" and the most powerful practitioners seek a balance between this and pure muscular "external power" that is both practically effective and philosophically pleasing. Warrior monks who cultivate can perform supernatural feats with a fluidity that surpasses that of a wizard, even if a wizard's magic is more complex.
Some holy men, monks and priests, cultivate not merely for physical strength or magical supremacy, but for religious reasons. Mortals cannot typically hear the gods. Most people are content to pray, hoping to be heard, and others make contact with celestials, beings that are connected to the celestial realm and can, so they claim, enter the heavenly courts. In order to speak, directly, with the gods, without an intercessor, one must become more like a celestial. Cultivation can achieve this. A mortal who has completely give their spirit to divine service is called a saint. Beings like einherjar and jiuwehu are associated with specific gods and are said to originate from devoted mortals. A mortal who has not achieved complete sainthood can still benefit from cultivation. Ascetics, when they are practiced enough, begin to wield divine magic more easily and may receive holy visions and see omens and signs more than the common layman.
Other cultivators are not so pure in motive. With the right practices and rare materials, one can cultivate longevity or even immortality. With that as the primary goal, rather than a side effect of magical training or monastic dedication, one can even seek immortality while pursuing a career in politics. The expense and difficulty of the practice, combined with an incredibly high failure rate, makes immortals understandably rare. Many powerful men who fear the end will turn to undeath instead. But the temptation of everlasting youth and the legends of transcendent immortals still drives many to seek the cultivator's ultimate alchemy.