The term 'scat syllable' is a type of vocal improvisation used in jazz music. The two crossword solutions for this term are 'DOO' and 'DUM, ' both of which are three-letter words. 'Doo' is an onomatopoeic word that imitates the sound of a drum beat, while 'dum' is an imitation of a bass drum. Together, these two words form a scat syllable, which is used to create a rhythmic pattern in jazz music.