Webster's Third New International Dictionary (NI3) defines pattern-word as, "(in) cryptology: a plaintext idiomorph". The same source defines idiomorph as, "a pattern of repeated letters in cryptography". Thus to a cryptanalyst any word with one or more repeated letters is a pattern-word. PROPER, STATE and MUUMUU are pattern-words, whereas THING, WORD and UNCOPYRIGHTABLE are not. In a simple substitution, monoalphabetic cryptogram, pattern-words can provide a method of quick solution where the cyrptogram is of the easy or medium-difficult variety. Thus a long word ending with the pattern -ABCC is very likely to end in NESS. The ending -DBBC is often -ALLY.
Pearce, Murray R.
Word Ways: Vol. 4
, Article 2.
Available at: http://digitalcommons.butler.edu/wordways/vol4/iss3/2