A coin described as BU means the coin is in Brilliant Uncirculated condition, meaning the coin has not been circulated and shows no signs of wear. A coin in BU condition will often have lustrous surfaces and brilliant colors, a perfect example being a $20 Liberty Gold Double Eagle BU in flawless condition and pristine luster.
Comparatively, an AU (Almost Uncirculated) coin is a coin that has been circulated but does not show significant wear, a great example is a $20 Liberty Gold Double Eagle AU coin. While an AU coin may have some light scratches or nicks, the overall condition of the coin is still considered to be very good.
So, in short, a BU coin is a brand new-looking coin that has never been circulated, whereas an AU coin is a pre-owned coin that shows some slight wear but is still in good condition.
