ARF – Average Reduced Frequency

a modified frequency which prevents the result to be excessively influenced by one part of the corpus (e.g. one or more documents) which contains a high concentration of the token. If the token is evenly distributed across the corpus, ARF and absolute frequency will be similar or identical. In comparison with ALDF (Average Logarithmic Distance Frequency), ARF is calculated from the frequency of the token, not distances between the tokens.

see also
ARF definition