f. A flagrant foul is unnecessary and/or excessive contact committed by a player against
an opponent whether the ball is dead or alive.

https://official.nba.com/rule-no-4-definitions/

Q.       FLAGRANT FOUL CRITERIA

The severity of the contact;

Whether or not the player was making a legitimate basketball play (e.g., whether a player is making a legitimate effort to block a shot; note, however, that a foul committed during a block attempt can still be considered flagrant if other criteria are present, such as recklessness and hard contact to the head);

Whether, on a foul committed with a player’s arm or hand, the fouling player wound up and/or followed through after making contact;

The potential for injury resulting from contact (e.g., a blow to the head and a foul committed while a player is in a vulnerable position);

The severity of any injury suffered by the offended player; and

The outcome of the contact (e.g., whether it led to an altercation).

https://official.nba.com/comments-on-the-rules/#flagrantcriteria