This term isn’t one which pupils are likely to need, because it belongs to an earlier era of language study when Latin and Greek were widely taught. It refers to the changes in the form of a word which alter the word’s grammatical function in a sentence - for example, walk, walking, walked, walks.
In Latin, there were many word endings (see inflection
) which were studied under this heading. However, the notion is of limited relevance for work on English, where there are few such word endings. These days, the term has been largely replaced by a more general conception of word structure: see morphology