Accidence is a grammatical term that is concerned with the study and usage of inflection. Accidence is sometimes used interchangeably with inflection, which is altering the form of a word to signify tense, gender, emotion, case, voice, or aspect. Prefixes and suffixes are frequently used as inflection in language. We see this in English where two words are spelled alike but when different syllables are emphasized have different meanings. Example "invalid" alternately means "lack of validity" or "physically handicapped depending on whether you emphasize the first or second syllable.