Coinbase

The transaction that created new Bitcoins would be referred to as the coinbase of each block.

The authors randomly sampled 100 coinbases from the last 20,000 blocks of the blockchain that by the time of the experiment had the highest block-height at 247,054.

Coinbase transactions have nothing to do with the US company of the same name. Coinbase is the required first transaction of every block and is the only transaction allowed to bring bitcoins into existence.