Sounsyy will surely be here shortly with references, but conceptually, materia in XIV is the crystallized aetheric energy of the memories and experiences of the person using the object that was converted into materia. That's what the whole spiritbonding thing is all about. With that in mind, I view materia largely as Warren does; it magically alters the piece of equipment into which its melded, making it lighter, faster, sharper, or what have you. There's a finite limit to how much particular pieces of equipment can be enhanced in this way, which is why there's a limit to the number of melds you can perform, and why too many melds destroys the materia (or, back in 1.x, the whole item). I seem to recall Mutamix or one of his associates explaining why that's the case; my recollection is that it has to do with the aetheric capacity of different materials, but I'll be the first to admit I'm probably misremembering that. 
The Freelance Wizard
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((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
