What is this called on the T? 0

[Yodelz]

[Coinbuf]

That is called a die chip or in some cases a retained cud if it occurs near the rim.  A die chip or cud is a small part of the die that breaks off and is imparted or left on the coin during striking.

[Just Bob]

Just a little point or two for clarity:

If a piece of the die breaks off, and is struck into the coin, it is a " retained struck-through" error. (Aka "struck-in" or "embedded.") It occurs when a coin is "struck through" a foreign object, in this case the piece that chipped off the die, and the object is struck into the surface, and stays there.

A " die chip," (or "cud," if it is at the rim) is when a piece of the die breaks off, and the planchet (coin blank) metal flows into the void left by the missing piece. This is what appears to have happened to your coin.

[Rummy13]

1 hour ago, Just Bob said:

Just a little point or two for clarity:

If a piece of the die breaks off, and is struck into the coin, it is a " retained struck-through" error. (Aka "struck-in" or "embedded.") It occurs when a coin is "struck through" a foreign object, in this case the piece that chipped off the die, and the object is struck into the surface, and stays there.

A " die chip," (or "cud," if it is at the rim) is when a piece of the die breaks off, and the planchet (coin blank) metal flows into the void left by the missing piece. This is what appears to have happened to your coin.

Thanks @Just Bob, I just learned something. I have a 2019 W Quarter that was graded and has a "struck Through" on it.