A miniature painting is a very detailed, very small painting. We're talking tiny, but exactly how tiny varies between miniature painting societies around the world. A rule many ascribe to is that to qualify as a miniature painting, it must not be larger than 25 square inches and the subject must be painted no more than one sixth of its actual size. So, for example, an adult head which is typically 9" wouldn't be painted large than 1½".
A traditional-style miniature isn't merely about size, but also the level of detail in the painting. It's the detail that differentiates a miniature from a small painting: if you look at it through a magnifying glass, you'll see extremely fine brushmarks with every detail scaled down and miniaturized. Techniques used include hatching, stippling, and glazing. Composition, perspective, and color are as important as in larger paintings.
