The term derives from minium, a mineral that was used to create red ink. In fact, it refers to writing with red ink that was used for the initial letters of sections and chapters of manuscripts.
The initial letters, which are often larger than the body text, were illuminated only when the remaining part of the text was completed. To facilitate the work, the copyist outlined, with a fine light line, the letter the illuminator would later design.
We find an example of this practice in the Librone I by Gaffurio (large format manuscript choir book), from which a detail of sheet 8v (f. 8V) is reproduced here.
The image is taken from the recent digitisation of this book and of Libroni II and III during a research project conducted by the Schola Cantorum Basiliensis.