Digram
A pair of two letters, the unit cryptanalysts use to read the statistical fingerprint of a language.
Digram frequencies differ sharply between languages. In English, TH and HE dominate. A digram-based cipher like Playfair hides individual letter frequencies but not the patterns between pairs, so counting digrams in the ciphertext reveals a great deal about the language underneath.
The longer the ciphertext, the clearer those digram frequencies become. Someone who knows the language can often guess which pairs belong together, and then the key is no longer needed to get at the content.