I consider it a painting when it is a work of pure color. My underpaintings usually consist of shapes and negative space and I work up from there in layers. Oil painting and chalk pastel are not that different for me process-wise, because both involve the gradual accretion of pigment.
In oil I tend to employ glazes to achieve luminosity and depth. I love the way oil paint moves on a canvas, and though I do also work with acrylics I just never enjoy the experience quite as much.
When painting with chalk pastel I scumble contrasting colors over one another to achieve visual dimension. Chalk pastel is glorious because of it's vividness. It is pure pigment, pure and simple. I love that it can be hard or soft, rough or smooth.