What is the half circle at the bottom of the key on a pro basketball court?

I think it started showing up in the last year or two.

What is the half circle at the bottom of the key on a pro basketball court?2006 nba draft

That is the border for what is now considered too close to take a charge. If a defender is inside that half circle, it doesn't matter how set he is, the offensive player can basically plow right over him without a foul because he is "too close to the basket"

What is the half circle at the bottom of the key on a pro basketball court?nba schedule ,nba teams

In the NBA, a charging foul cannot be called if the play occurs within a 4' (1.22m) radius around the center of the basket.
It's a 3-second zone for the defense.
this arc is for the following:

inside that circel u can not draw a charge on any attacker!

if you are in it, u shall be called for the foul!

if both your feets are out of it, u can draw the charge on the attacker!

that is called fill in the lane!
Stingo got it - if a defenders feet are inside the arc, he is considered under the basket and cannot take a charge.
3 second defensive zone