Final answer:
In swing band arranging, 'soli' is a passage for a specific section of the band to play harmonized or in unison, highlighting their cohesive sound.
Step-by-step explanation:
In the context of swing band arranging, a "soli" refers to a section of music where a particular section of the band (such as the saxophones, trombones, or trumpets) plays a passage in a harmonized or unison style together without accompaniment from the rest of the ensemble. It is a showcase for the section to demonstrate its cohesive sound and blend. The term 'soli' can be seen as a variation of 'solo', meaning that instead of one person, a section of the band is highlighted.