The Six Principles of the ICZN: Binomial nomenclature.

Before Linnaeus, the scientific naming of species was very messy. There was no standard way of going about it. Names were often long descriptive phrases in Latin (which was the standard scholarly language of the time). For example, a species of Geranium was written like the following:

GERANIUM pedunculis bifloris, calycibus pyramidatis angulatis rugosis, foliis quinquelobis rotundatis.

Needless to say, if names are to act as a reference system the above is extremely cumbersome. At the time of Linnaeus, an unimaginable amount of species were being discovered over a relatively short period of time. A simpler system was badly needed, and binomial nomenclature filled this need. Linnaeus consistently used binomial nomenclature for his Systema Naturae and it was popularized further by like minded workers of the day in Zoology and Botany. When the first code of zoological nomenclature came into use, the Strickland Code (1845), the use of binomial nomenclature was made a rule, and has been retained through the subsequent codes, up to today's 4th edition of the ICZN.

It is stated thus -

The scientific name of a species, and not of a taxon of any other rank, is a combination of two names (a binomen), the first being the generic name and the second being the specific name. The generic name must begin with an upper-case letter and the specific name must begin with a lower-case letter.

(From ICZN, Article 5)

The Principle of Binomial Nomenclature provides for a stable and easily recognizable scientific species name. Only a species name is made of two names, the first capitalized and the second lowercase. The Code gives further corollary statements on subspecies trinomens, higher ranks than species being of only one word, but this follows the general statement of the principle above. Its our starting point in zoology. With a two word name where no other names are two words, there's no room for ambiguity.

With the possible ease of a 1 word system, a person may wonder why Linnaeus didn't just use that? The answer is, in a monomial system one would quickly run out of names for all the animals. With a binomial system, the number of combinations is nearly infinite (when I try to calculate it, my graphing calculator gives me an overflow error for n=1 million; it gives me overflow for 10,000 for that matter).