A magic burst is the effect created when a spell, or certain other abilities, is successfully cast upon a target, within a specific window of time after a Skillchain has been performed. Effects that can create a magic burst, are spells, bard songs, ninjitsu, Blood Pacts from an Avatar, or a wyvern's breath attack. In order to create a Magic Burst, the element of the spell or effect must match the element of the Skillchain. The spell must finish casting within five seconds of the Skillchain's animation to produce a magic burst.
Characters can magic burst between skillchains or at the end of a skillchain (which must involve two or more Weapon Skills). To magic burst in between Skillchains, the spell must be completed after the first Skillchain has been closed, but before the next Weapon Skill is activated. Additional Weapon Skills after the Skillchain will interrupt the 'window' for magic bursting.
More than one magic burst can happen at the same time (i.e. two different people can cast two different spells, as long both spells are the correct elemental type). They will not interfere with each other. Characters can also magic burst a Skillchain that they helped create.
With very careful timing, a character can magic burst multiple times during the same Skillchain if the spells have short enough casting times. White Mages can double or even triple burst (with perfect timing and Fast Cast) Light with Banish III, Flash, and Holy. Red Mages can easily double-burst or higher during Chainspell.