Define an un stable approach?
Seriously???
Any approach that does not meet the criteria for a Stable approach. While most operators have their own specific criteria, In General...
Prior to 1000' in IMC or 500' in VMC:
The aircraft is on the correct flight path;
Only small changes in heading/pitch are necessary to maintain the correct flight path;
The airspeed is not more than VREF + 10kts indicated speed and not less than VREF;
The aircraft is in the correct landing configuration;
Sink rate is no greater than 1000 feet/minute; if an approach requires a sink rate greater than 1000 feet/minute a special briefing should be conducted;
Power setting is appropriate for the aircraft configuration and is not below the minimum power for the approach as defined by the operating manual;
All briefings and checklists have been conducted.
Specific types of approach are stabilized if they also fulfil the following:
ILS approaches must be flown within one dot of the glide-slope and localizer;
a Category II or III approach must be flown within the expanded localizer band;
during a circling approach wings should be level on final when the aircraft reaches 300 feet above airport elevation; and,
Unique approach conditions or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.
An approach that becomes unstabilised below 1000 feet above airport elevation in IMC or 500 feet above airport elevation in VMC requires an immediate go-around."