A phonological—or similarity—neighborhood consists of a set of similar-sounding form-based representations that are activated in memory based on how closely those word-forms resemble the stimulus input. A neighbor of a target word is determined on the basis of the addition, deletion, or substitution of a phoneme in any position of the target word. The collection of neighbors computed in this manner is commonly referred to as the phonological similarity neighborhood. The number of neighbors is referred to as neighborhood density. By computing the mean of the frequency of occurrence of the neighbors, one obtains a value referred to as neighborhood frequency.