usually used to signify how artsy
you really are.
A bleed mount is when a print is made without cropping the borders between the negative and the negative carrier. So instead of having a print that is clearly defined with a solid (rectangular or square) border, you have a print with these thick black edges and extra shapes and extra whatnot.
I hope this definition makes sense, a print made as a bleed mount is immediately recognizable, but the difficultly comes from describing it...