In The Scarlet Letter, what is Dimmesdale's "A" a symbol of?

Arthur Dimmesdale's letter on his chest is a manifestation
of his guilt since for seven years he has kept his secret sin within his heart.  As he
has told Roger Chillingworth in Chapter X, some people conceal their sin because they
yet possess a zeal for God's glory and man's welfare:

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...they shrink from displaying themselves black
and filthy in the view of men; because, thenceforward, no good can be achieved by them;
no evil of the past be redeemed by better
service...."

Thus, the hidden
mark upon his bosom is this "black and filthy" conscience that he possesses.  Even when
he goes in the night to stand on the scaffold, Dimmesdale cannot bring himself to
confess. And, his watching Hester suffer alone for the sin which he has committed with
her tortures him until his flesh manifests this terrible
guilt. 

In Chapter X when Chillingworth seeks to elicit
from Dimmesdale a confession, knowing that the minister's sickness is a manifestation of
his spiritual illness, he tells the minister that some black herbs have grown out of a
man's heart whose hideous secret he did not reveal. But, Dimmesdale is unable to reveal
"the secrets that are buried in the human heart." So, eventually, his guilt rises to the
surface of his flesh and manifests itself. This manifestation of his guilt is what gives
the evil Chillingworth such delight:

But, with
what a wild look of wonder, joy, and horror!  With what a ghastly rapture...Had a man
seen old Roger Chillingworth, at that moment of his ecstasy, he would have had no need
to ask how Satan comports himself, when a precious human soul is lost to heaven, and won
into his kingdom.