Neighbour

Neighbours of a word are all other words that only differ in one or more segments. Normally only orthographic neighbours are considered. The most common definition of word neighbours would be all other words of equal length that only differ in 1 letter from the original word.

Substitution neighbours

The classical definition, where one or more characters in the word are substituted. Normally only a single substitution is allowed. When two substitutions are considered the special case of 'adjacent transposition' has to be excluded. Examples are 'these'/'those',

Deletion neighbours

One or more characters can be deleted from the word. Examples 'words'/'word'

Insertion neighbours

One or more characters can be deleted

Adjacent transposition neighbours

Two adjacent characters are transposed, for example 'trial'/'trail'. This is not considered a double substitution.