1) Formal language 2) Letter pair 3) Two-letter cluster 4) Written word 
A bigram or digram is every sequence of two adjacent elements in a string of tokens, which are typically letters, syllables, or words; they are n-grams for n=2. The frequency distribution of bigrams in a string are commonly used for simple statistical analysis of text in many applications, including in computational linguistics, cryptography, spee...
[n] - a word that is written with two letters in an alphabetic writing system
noun a word that is written with two letters in an alphabetic writing system