A blazon is a formal written description of a shield or coat of arms. The language used in such descriptions is also known as blazon. Blazons follow a very precise formula which allows people to reproduce shields from written descriptions alone, and they can be quite simple, or extremely complex. Students of heraldry are familiar with blazon, although this complex and arcane language can seem totally incomprehensible to ordinary civilians.
Many of the terms used in heraldry are French in origin, including the term “blazon” itself, because French was once the formal language used by English clerks, and heraldry originated in England. The common practice in blazon of placing adjectives after nouns is also a holdover from the French; hence a phrase like “a field azure” means “a blue background.”
When describing a coat of arms, a precise formula is used. First, the “field” of the shield is described; the field is simply the background, and it may have a tincture, or color, or a fur, or pattern. There are seven acceptable tinctures in modern heraldry: or (gold), argent (silver), azure (blue), gules (red), purpure (purple), sable (black), and vert (green). There are eight different furs, including ermine, a pattern based on the fur of the stoat. The shield may also have divisions created by a band, stripe, chevron, combination of bars, and so on, in which case the divisions are described from top to bottom and left to right.
Following the shield, the primary charge, or figure, on the shield is described. A charge can take any form, although often mythical beasts are used. If secondary charges appear, these are also described. If the bearer is entitled to things like supporters, figures which hold up the shield, these are also described in the blazon. If a crest tops the shield, this is included, and the motto at the bottom of the shield will also be described.
It may interest you to know that when artists draw shields from the description in a blazon, they can turn out looking very different. The shape of the shield, for example, is not terribly important in most cases, and it may be manipulated to suit a particular application.
It is the combination of elements which makes a shield unique. For example, a blazon might describe a coat of arms as "argent a bend sinister vert overall a griffin rampant purpure," which is to say that the shield is silver with a green stripe which runs from right to left, and a purple griffin in battle position covers the shield. There are a number of ways in which this blazon could be represented pictorially, but they would all be recognizable.
When someone applies for armorial bearings, if his or her application is accepted, a formal blazon will be written up to describe the shield. Commonly a pictorial representation will be drawn as well, and these formal documents are often kept as family or company heirlooms.