BU is BRILLIANT UNCIRCULATED...the most misused term in numismatics. For me, it implies the coin surfaces are completely original, bright and full of mint luster with NO toning or coloration. Any MS coin from MS60 up can be BU, but it MUST have those characteristics or it's just uncirculated.
I see people on
ebay using the term BU incorrectly about 90% of the time. There is nothing brilliant about a brown Lincoln or a toned coin.
Lincoln Cent Lover!VERDI-CARE™ INVENTOR
https://verdi.care/