Mana is a power source often found in the Tales series, often used in magical actions or when referring to health, or life force. It is an important theme in both Tales of Phantasia and Tales of Symphonia.
Tales of Phantasia
Mana is the power born from the Great Tree Yggdrasill and supports Aselia. Mana provides the basis of both magic and Spirits, and it is used as a power source in magitechnology. Unfortunately, mana is not limitless; overuse in the past had killed Yggdrasill, leaving magic and Spirits, as well as the technology derived from them, lost in the present. Cress Albane and Mint Adenade rediscover mana when flung to the past, where Yggdrasill still lives, though badly weakened. As the revived Dhaos now plans to annihilate their present, they decide that magic is the power they need to combat him, though such arts are lost in their time. Conversing with the Spirit Martel who guards Yggdrasill, they learn that something has been draining mana faster than Yggdrasill can create it, slowly killing the tree.
They eventually discover that the advent of magitechnology is what happened. Magitechnology drains mana even more than magic does, and its use would eventually kill the Tree. Midgard's magitech weapon is the definitive example, with Martel visibly worried after just one shot with it. In Tales of Phantasia: The Animation, this is additionally represented by Claus F. Lester's Spirit summon Undine unable to hold her physical form immediately after the shot is fired. By using the power of the Unicorn, Mint manages to heal the Tree well enough to have it survive even into their future, which is where they head after Dhaos flees there. Mana is still plentiful in the future, though magitechnology also exists there. At the end of the game, at Dhaos's defeat, Mint erects a barrier around Yggdrasill so that too much mana cannot be drawn from it, allowing the Tree to form a Mana Seed to send back to Dhaos's Derris Karran home.
Tales of Symphonia
Mana is the basis of all life in Sylvarant and Tethe'alla. Crops wilt and droughts come when the world is declining and mana becomes scarce. The entwined worlds of Sylvarant and Tethe'alla work like an hourglass: mana flows to the flourishing world and out of the declining world, but when the Chosen ends his or her Journey of Regeneration, the hourglass is turned over, causing mana to flow into the declining world. It is also the source of magic and Summon Spirits, with the former being how elves and those of elven blood cast their spells.
During the game, it is discovered that mana plays a role in containing and protecting the Great Seed. Mana links, the mana flowing between Summon Spirits of opposing elements, forms a sort of cage around the Great Seed, keeping it between the two worlds. However, when Sheena makes a pact with both Undine and Volt, who have opposing elements, at the same time, neither is asleep, and the mana link is broken. The continued breaking of these links is what causes the Giant Kharlan Tree to germinate in a mutated form, wreaking havoc on the land. The protagonists use Rodyle's Mana Cannon to blast it with mana from the opposing Summon Spirits, neutralizing the imbalance. During this rampage, the tree destroys the city of Palmacosta. The fact that Marta Lualdi's mother died in this disaster is the source of her animosity toward Colette Brunel in their first meeting in Tales of Symphonia: Dawn of the New World.
Mana did not originally exist in the world in Tales of Symphonia. When the comet Derris-Kharlan first came to the world, and the elves transplanted the Giant Kharlan Tree, mana became part of the world, creating life and sealing it from Niflheim, the demonic realm. Since mana is toxic for demons, few of them remains in the human world, and those who still exist have their power extremely weakened. In the sequel, the Summon Spirit Ratatosk, at the end of the game, decides to rewrite the world's laws so that life itself would not be so dependent on mana. Doing so would allow the excess mana to guard against Niflheim in place of Richter Abend, who in the meantime sacrifices himself to guard the barrier.
Tales of Vesperia
Rita Mordio comes up with a theory that they can condense aer into mana, which she describes as a substance closer to matter than aer. As mana is more stable than aer, it would combat the Adelphagos threat. To do this, they try to convert the Entelexeia and eventually all the world's blastia cores, which are apatheia, into spirits to handle the resulting mana.
Tales of the World: Radiant Mythology
Mana is born from the World Tree and flows out into the world, providing it with life. Without mana, the world itself would die. The places where there are large concentrations of mana are where the world is strongest, though magitechnology can weaken it. A Descender's power also fluctuations with the strength of mana and the World Tree it comes from. On Terresia, the Ailily, Doplund, and Gavada are the three locations best protected by mana, though the advent of magitechnology brought by Aurora has weakened Gavada's mana. It is the Devourer's eventual goal to eat the world, but the world's mana prevents that, and so its agents seek ways to weaken mana.
Kanonno reveals that all mana eventually returns to the World Tree, but on her world of Pasca, its inhabitants had done something that prevented the mana from returning, resulting Kanonno to give her world up to the Devourer in order to save it from complete annihilation. Mormo says that each world's mana has its own distinctive smell, and in the Orphic Maze in particular, he cannot smell Terresia's mana, showing that the Orphic Maze's origin is not Terresia.