No, flashback is not an ability that can be activated at instant speed.
Yes, you can cast a sorcery spell at instant speed by using a flashback ability.
Yes, you can activate a planeswalker ability during your opponent's turn if the ability can be activated at instant speed.
Yes, in Magic: The Gathering, you can activate creature abilities on your opponent's turn if the ability can be activated at instant speed.
the rate at which an object is moving at a given moment in time.
Instantaneous speed is speed at any instant in time.
Instantaneous speed is speed at any instant in time.
The speed of an object at any instant in time is its instantaneous speed.
Speed of an object at one instant of time is the object's instantaneous speed.(Not velocity.)
Instantaneous speed-the speed of an object at any instant time. When you ride in a car, the instantaneous speed is given by the speedometer.
instantaneous speed
It is the speed or velocity at a particular instant.
Interrupt is an obsolete game term, and any card that had it, is now an Instant, or an ability that can be played at instant speed. Counterspell was originally an Interrupt, as were many other counter cards. Any of them, that have been reprinted, will now show as an Instant and even if a card has not been reprinted, its Oracle wording will now show it as an Instant.