The Trinity of Magic is the triangle that determines effectiveness between the types of magic used. The triangle starts off with anima, or elemental magic, at the top. Going counter-clockwise, anima magic is effective against light magic, the bottom left of the triangle. Then, light magic is effective against dark magic, the bottom right of the triangle. Finally, dark magic is effective against anima magic, back to the top of the triangle. It's a continuous loop between the three magic types, with one other in the middle of the triangle: the Staff. Staves do not have any kind of magical affinity to it and is not used in attacking units, therefore it is excluded from the triangle, and are not strong or weak against any other magic.
Additionally, a Trinity of magic exists amongst anima magic. Fire magic (Fire, Elfire, Arcfire, Bolganone) is effective against wind magic. Wind magic (Wind, Elwind, Arcwind, Rexcalibur) is effective against thunder magic. Thunder magic (Thunder, Elthunder, Arcthunder, Thoron) is effective against fire magic.
If a magic tome is effective against another in battle, the stronger magic gets +1 damage and +15 hit rate, and the weaker magic gets -1 damage and -15 hit rate.