Two words of the same length are said to crash if they have matching letters in one or more positions, such as tIger and pIous, or roTuND and atTeND. A symmetric crash group is a set of words in which (1) each word crashes exactly n times with each other word, (2) every letter participates in a crash, and (3) each letter is used exactly m times in a given position.
Eckler, A. Ross
"Symmetric Crash Groups,"
Word Ways: Vol. 11
, Article 13.
Available at: http://digitalcommons.butler.edu/wordways/vol11/iss4/13