MAC doesn't really define their colors. The terms frost, matte, veluxe, etc. refer to finishes, which is basically the way the shadow looks. Mattes are matte, lustres are sparkly, frosts are frosty/shimmery, etc.

Here's the breakdown from MAC:

FROST
An iridescent shine that adds a highlight to any colour.

LUSTRE
Smoothly pearlized; intensely frosted with deluxe ultra-fine "conditioned" finish.

MATTE
Hi-colour pay-off in a no-shine matte finish.

SATIN
Pure colour in a satin-style finish. Provides a soft subtle non-frost, light-refractive sheen.

VELUXE
Superfine pigment rich and very luxe. Supple to apply. Provides silky smooth matte finish.

VELUXE PEARL
Vibrantly-toned. Velvety soft. Shimmeringly metallic. Veluxe eye shadow, now overlaid with high-shine pearl.

VELVET
Soft look finish with high-colour intensity. Has a plush velvety look and feel.