It is, as Oxcomper suggests above, called an 'initial'. Here's what The Oxford English Dictionary says as regards this particular meaning of the noun 'initial'...
"In old manuscripts, frequently, and in modern publishing, occasionally, the initial letters at the heads of paragraphs or sections made large and more or less ornamental."
Thank goodness i got it right, working as I do in a Library with many illuminated manuscripts it would have been embarrasing to say the least if anyone who knows me saw a wrong answer!
I don't think anyone is disputing the correctness of 'versal', Bricro. The thing is that Chambers is not nearly as extensive as The Oxford English Dictionary and it points out that 'initial' is correct, too. A good case might also be made for Nilya's 'rubric', which specifically refers to a red-coloured opening letter but one which may be (quote) "otherwise distinguished". All three are just fine!