Adumbration - n, 1. The act of making a shadow or faint resemblance.
2. A faint sketch; an imperfect representation of a thing.
3. In heraldry, the shadow only of a figure, outlined, and painted of a color darker than the field.
By all accounts I was able to find, adumbration is directly tied to the art of "blazoning, assigning, and marshalling a coat of arms." It is included in A Glossary of Terms used in Heraldry (Parker & Company, 1894) with this definition:
"Adumbration, or Transparency: the shadow of a charge, apart from the charge itself, painted the same colour as the field upon which it is placed, but of a darker tint, or, perhaps, in outline only. The term belongs rather to the romance of heraldry than to its practice, and is imagined by the writers to have been adopted by families who, having lost their possessions, and consequently being unable to maintain their dignity, chose rather to bear their hereditary arms adumbrated than to relinquish them altogether."
And again in Pimbley's Dictionary of Heraldry (Arthur Pimbley, 1908).
"Adumbration - A figure on a coat of arms traced in outline only, or painted in a darker shade of the same color as the field on which it is represented. Families who had lost their estates, but not their armorial bearings, are said to have occasionally adopted this method of indicating their peculiar position."
I tried to find a visual representation. I found plenty of heraldry photos, but this was all I was able to find that was particular to adumbration. It'll give you at least some idea of what adumbration might have actually looked like.