The spontaneous creation or variation on an established theme within an artistic work is known as .A rhythm that emphasizes the off or irregular beats is known as .

The spontaneous creation or variation on an established theme within an artistic work is known as improvisation. This is a common practice in many forms of art, including music, dance, and theater. It involves creating or changing an aspect of the work on the spot, often in response to the immediate situation or environment.

A rhythm that emphasizes the off or irregular beats is known as syncopation. This is a musical term that refers to a disruption or interruption of the regular flow of rhythm. It's a way of playing or singing notes at times when they're not expected, or not playing or singing notes when they are expected. This creates a rhythmic surprise or "off-beat" feel.