English word for a decorated opening letter

What is the English name for a highly decorated letter at the beginning of a paragraph, mostly found in ancient texts?

For example, in the following Opening folio of the Hengwrt manuscript of the Canterbury Tales, the letter W - the first letter in the page - is beautifully painted.

They are known as Initials or Drop Caps, depending on whether the large first letter rises above the text or, as in your image, goes down below it. This is from the Wikipedia entry linked earlier:

an initial is a letter at the beginning of a word, a chapter, or a paragraph that is larger than the rest of the text.

More specifically, the decorated initial is Illuminated. With the addition of the fancy borders, your example as a whole would be described as an Illuminated Manuscript.