It's called a "Strike Through Error" "Missing Element Coins" or "Filled Dies". Another name is "Grease Coin". Grease is added to the machine allowing quicker production. Some of the grease ends up on the die resulting in lightly pressed or missing lettering. Fairly common error. It's worth about a $1.00 to the collector that wants one. The coin price would be worth more if it was an extreme type of error.