The term 'unaccented syllable' refers to a syllable that is not stressed or accented in a word. An atonic syllable is a type of unaccented syllable, which is a syllable that does not have any stress or emphasis on it. This type of syllable is often found in words with multiple syllables, such as 'DICTIONARY' or 'ELEPHANT'. Atonic syllables are usually unstressed and can be found in the middle of a word or at the end.