In a written or published work, an initial is a letter at the beginning of a work, a chapter, or a paragraph that is larger than the rest of the text. The word is derived from the Latin initialis, which means standing at the beginning. An initial often is several lines in height and in older books or manuscripts, sometimes ornately decorated.

In illuminated manuscripts, initials with images inside them, such as those illustrated here, are known as historiated initials. They were an invention of the Insular art of the British Isles in the eighth century. Initials containing, typically, plant-form spirals with small figures of animals or humans that do not represent a specific person or scene are known as "inhabited" initials. Certain important initials, such as the B of Beatus vir ... at the opening of Psalm 1 at the start of a vulgate Latin psalter, could occupy a whole page of a manuscript.

These specific initials, in an illuminated manuscript, also were called Initiums.

The classical tradition was late to use capital letters for initials at all; in surviving Roman texts it often is difficult even to separate the words as spacing was not used either. In the Late Antique period both came into in common use in Italy, the initials usually were set in the left margin (as in the third example below), as though to cut them off from the rest of the text, and about twice as tall as the other letters. The radical innovation of insular manuscripts was to make initials much larger, not indented, and for the letters immediately following the initial also to be larger, but diminishing in size (called the "diminuendo" effect, after the musical notation). Subsequently they became larger still, coloured, and penetrated farther and farther into the rest of the text, until the whole page might be taken over.