It seems that everyone has posted a pretty common sense approach to it all. Well, maybe not the guy whose head exploded. Anyways, I want to put forth an argument as the Miniature Devil's Advocate.
A miniature is anything that is a smaller sculptured representation of all of the naturally occuring dimensions of a particular subject. A "model" which is 90% of the average size of a knight in armor shouldn't be considered a miniature since a knight could naturally occur at 90% of the average dimensions. Since a knight cannot naturally be 5% of the size of an average knight, a 1/20th scale model of a knight would be a miniature.
It seems obvious, but there are some counterintuitive examples.
For instance, a 50% scale sculpture of a human might not be considered a miniature since there are humans that can be properly proportioned and in those dimensions. (variations of dwarfism)
A model of Paint Horse that is 1/10th the dimensions of the average Paint Horse might not be considered a miniature, as there are horses that small. Instead of calling it a Miniature Model Horse, it might be more accurate to say it is a Model of a Miniature Horse.
Defining a miniature as anything smaller than a 1:1 ratio is a poor definition since it cannot be applied in practice without defining the values. In other words, you can't say something is a miniature until you specify what 1, or 100%, is. If the specific knight you are making a model of is 73 inches tall you have defined 100% with a definite value, not a relative one. Thus, a sculptor who made a marble statue representative of the specific knight that was only 67 inches tall has created a miniature.
Fantastical creatures are a bit more difficult. Who is to say how tall a Gnome is? Unless you have documented in some way what size your subject is supposed to be you cannot state that the model you have sculpted is a miniature.