Moonlapse said the word: "innate." In my opinion, "talent" is an inner quality of a person, a "gifted" one with a skill (this talent) which enables this person to excel at something more easily.
I take the example of language teaching because this is what I do. I have a class with 20 students; the theory is the same, and so is the practice; however, there're some people who have a talent for languages and can pick up words and expressions quite easily, whereas others just can't, even if they spent their whole life studying.
I guess it works similarly with music. I would have a talent for music if I could easily create some nice music, if I had a sensibility towards songs, if I could react to them, feel them; I would have talent if I could grasp musical stuff somewhat easily, if I was sitting with my guitar on my lap and coming across nice melodies...
Talent cannot be measured. However, by means of theory and practice (exercise), listening to music, you can polish that talent, train and express yourself a better way. But this wouldn't change your inner quality; you could have as much theory and as much practice... if you're not talented, then it will take you a much much longer way to create something, if you can manage!