Circle of confusion.
Simply put:
→ A circle of confusion is created when an object is not in the focus plane.
→ The lack of focus creates a blur circle centred on 'where it could have been in focus'.
→ This means in turn that a circle of confusion is created either in front of the focus plane or behind it.
Use:
→ Circles of confusion, both in front and/or in the back can be used for effect.
→ Circles of confusion created from a point of light is referred to as 'Bokeh'.
Circle of confusion as illustrated on Wikipedia:

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