As Webster 1913 says, a horizontal
band across a shield
. In theory, but never in practice, it occupies a full third of the field
; even if it was the only thing on the shield it would be a little less than that, and normally it is a stripe
amid other devices.
Two bands across a shield are called bars. There cannot be just one bar. A stripe narrower than a fess or bar is a barrulet. This is rarely found on its own, but rather in pairs: two barrulets above, and two below, for example, or six barrulets in all, grouped in pairs. One such pair is called a bar gemel. (So a shield containing six paired barrulets contains three bars gemel.)
Most such divisions of the shield can be made with other kinds of line as well as straight lines: wavy, indented (zigzagged), and so on. One such line is embattled, that is having square wave teeth like battlements. An oddity of the fess is that, if embattled, only the upper side is embattled.
A field divided in half by a horizontal line is described as 'per fess' or 'party per fess'. A field divided into, say, eight equal horizontal parts is called 'barry of eight'.
An individual object oriented horizontally is described as fesswise. A group of objects arranged from left to right are described as being in fess.
An unusual way a fess can be borne is as a fess wreathed, which is to say represented like the coils of a thick rope.
Also slang for confess
, often in the expression fess up