Final answer:
The two essential aspects of jazz are swing and improvisation. Swing is characterized by a rhythm that includes polyrhythms and syncopations, and improvisation involves spontaneous musical solos.
Step-by-step explanation:
The two essential aspects of jazz according to the text are swing and improvisation. Jazz is a musical style that emerged from African American communities, with its roots in genres such as blues and ragtime. Its distinctive feature is its rhythm, which includes elements like polyrhythms and syncopations, giving it a characteristic swing.
Furthermore, jazz is well-known for its improvisational nature, where musicians deliver spontaneous solos that reflect their individual expression and reaction to the flow of the music.
This ability to improvise within the framework of the tune's harmony and melody is one of the hallmarks that sets jazz apart from other musical forms.