How does one reference an unspent transaction output? In a bitcoin transaction the previous unspent transaction outputs are referenced.

How are these referenced?


  Do outputs have unique reference or maybe as
  
  unique transaction reference X output recipient's address (its public key)
Each transaction output is uniquely identified by an outpoint. Outpoints consist of


txid: The unique transaction ID of the transaction that created the output. The txid is the 32-byte sha256d hash of the transaction. 
index: The output index (vout) that marks the position of the output in that transaction. The output index starts counting at zero and is a 4-byte unsigned integer.


As an arbitrarily picked example, the outpoint 

e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978:1


refers to the second output of the transaction e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978 worth 0.001 BTC which was sent to address 1La27vpuDDr9NR1aZ6GpxoJyNYmHT62nRb.