How does one reference an unspent transaction output?

Question

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)

Answer 1

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.