Expressive timing refers to the musical phenomenon whereby a performer introduces subtle temporal nuances to an otherwise metronomic ("perfectly" timed) interpretation.
For instance, a pianist might introduce a slight ritardando (not called for explicitly in the musical score) at the end of a phrase to convey a structural event (in this case, a phrase ending).
Expressive timing has been shown to operate in different musical styles.
It has also been shown empirically that simple rhythms are often performed differently from how they are notated.
The following figure graphs timing data for a short phrase performed by John Coltrane ("Like Someone in Love" 2:42–2:45.