A non-essential dissonance
- is a non-chord note that temporarily replaces a chord factor
- usually occurs on a strong beat at the beginning of a new chord/sonority
- requires preparation
- is more often referred to as a suspension.
Dissonances are called non-essential because they don’t determine the identity of the chord, as they don’t change the type of the chord.
For more information see my essay on essential dissonances, in which I also elaborate on the difference between non-essential and essential dissonances.