noun A coding system that uses (typically variable-length) codes that are distinguished by their "prefix property", which requires that there is no code word in the system that is a prefix (initial segment) of any other code word in the system.
A prefix coding can use a code with code words {9, 55}, as it has the prefix property. However, a code consisting of {9, 5, 59, 55} is not allowed, as "5" is a prefix of "59" and also of "55".