Built up edge (BUE) is the accumulation of material against the rake face of the cutting tool.
https://www.youtube.com/watch?v=-TfNouJbJrg
Previously:
https://www.homemadetools.net/forum/...793#post118983
https://www.homemadetools.net/forum/...794#post124360
https://www.homemadetools.net/forum/...793#post118983