nonsense syllables in traditional music; rhythmic filler words; non-lexical vocables
In traditional Japanese music, these are syllables or words (like 'dokkoisho' or 'sore') inserted for rhythm, not for meaning. The term can also refer to similar filler words in folk songs or work songs.
In folk songs, hayashikotoba are used to keep the rhythm.
'Dokkoisho' is a well-known hayashikotoba.