Can you really place a definition on a word, though? Considering that words are merely verbal shorthand to refer to concepts which people intuitively add meaning to based on their experiences, any definition on a word is subject to change based on the listeners experiences with the concept which is being referred to.
The easiest example is defining an apple. We can say it's a fruit, we can describe its taste, its colour, its shape,... but the only way someone can truly understand what an apple is, is by actually seeing, smelling, feeling, and tasting an apple.
Well, with the obvious exception that not everyone is able to see, smell, feel, and/or taste, so even that way of defining an apple is tricky.
To come back on topic, yuri, as a genre, is therefore defined based on one's own expectations of what a work in the yuri genre ought to contain, and subsequently different people have different concepts of what the yuri genre is.
Though it might be forward of me, I'd offer my suggestion that at the very least, the yuri genre concerns itself with people of the female persuasion and their interpersonal relations with each other.
