The word E is the shortest English neologism and is a Spivak pronoun meaning he or she. It is also a common abbreviation for the drug Ecstasy.
e is a mathematical and scientific constant, referring to the base of the natural logarithm.
In gangster rap culture, G is a valid word, a contraction of "gangster", used as a term of respect.
There are also various single letter words used as abbreviations in text-speak and related uses: