Brun is a medium-dark, taupe brown with neutral-to-warm undertones and a matte finish. It's a long-time permanent staple in the MAC range, and I find it is a chameleon--it can look ashier against cooler tones and just barely warm-toned when paired with warmer shades. It had a soft, blendable consistency that wasn't too firmly nor too softly pressed in the pan, which yielded opaque pigmentation. It stayed on nicely for eight hours on me before fading visibly.
FURTHER READING: Formula Overview, which is a detailed review of the range as a whole.