Satoshi's paper says:
Once the latest transaction in a coin is buried under enough blocks, the spent transactions before it can be discarded to save disk space.
What are spent transactions?
Balances in Bitcoin are stored in Unspent Transaction Outputs utxo. UTXO can be used as inputs when a transaction is created. They can only be spent completely, while the resulting transaction creates new ones in the process, redistributing the balance to the recipients of the transaction. Spent transactions refers to Transaction Outputs that have been expended.