A brevigraph is a symbol that represents two or more letters in a word. Brevigraphs were used regularly in old books and manuscripts. The symbols are generally not available in any standard character set, including Unicode.

See also tittle and abbreviation.

When you go to your local Renaissance Festival, and see a sign saying "Ye Olde Tavern" or some such, that Ye is, in part, from the brevigraph "Y with e above" which stood for the.

Sometimes a brevigraph manifests as a diacritic over a letter. In Old English, a "y" with a mark of some sort over it can stand for the, thou, or that. An "e" with a short bar over it can stand for the letter combination en or em. (from http://xml.coverpages.org/mah-dual-57.html)

Other times an odd symbol is used, such as the sort of a backward c used in Old Norse for "con" at the beginning of a word. (see http://www.hum.ku.dk/ami/handbook/chapter4.html for more examples with pictures).

If you do a google search on brevigraph, you will primarily get discussions on how to encode them so as to maximize understanding and minimize data loss. Usually it looks something like this example from the Text Encoding Initiative (TEI) at http://www.tei-c.org/P4X/PH.html. This shows the word "persone" written with the leading "per" replaced by a specific brevigraph,

<abbr expan="per">&p-underbar;</abbr>sone 
Generally, an encoding for the various brevigraphs must be created, like this one from the Women Writers Project at http://www.wwp.brown.edu/encoding/training/Entities/wwpspec.ent.html :
<!-- WWPabb: special brevigraphs and superscripts, etc., for abbreviations -->
<!ENTITY ysupe  SDATA "{the}">  <!-- y with a letter 'e' above it, for 'the' -->
<!ENTITY ysupt  SDATA "{that}"> <!-- y with a letter 't' above it, for 'that' -->
<!ENTITY ysupu  SDATA "{thou}"> <!-- y with a letter 'u' above it, for 'thou' -->
<!ENTITY wsupt  SDATA "{with}"> <!-- w with a letter 't' above it, for 'with' -->
<!ENTITY wsuph  SDATA "{which}"><!-- w with 'h' above it, for 'which' -->
<!ENTITY iumlig SDATA "{ium}">  <!-- u with dot over left leg for i, 
                                              longer right leg for m -->
<!ENTITY pmacr  SDATA "{pre}">  <!-- p with a macron, for 'pre' -->
<!ENTITY phook  SDATA "{pro}">  <!-- p with a hook through the descender, 
                                              for 'pro' -->
<!ENTITY pbar   SDATA "{per}">  <!-- p with a bar through the descender, 
                                              for 'per' or 'par' -->
<!ENTITY qthorn SDATA "{quoth}"><!-- a blending of 'q' and 'thorn', 
                                              for 'quoth' -->