As David observes, neither Energy Interoperation nor the TEMIX profile define a transaction. The TEMIX profile defines the transaction service only as "The core services to reach agreement"

In section 4.1, the Transaction Facet is defined as "for expressing transactions."

The only thing that looks like a definition is in the enumeration of Facets

Transaction A Transaction is created by the Market to record a contract when a tender to buy and a tender to sell are matched. Both parties are notified of Transaction¬¬¬ creation.
See Section 10 Transaction Facet

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Removing EmixBase and associated complexity resulted in a simplified definition of EiTransaction. The same information is present with the enhancement that the relevant EiTender is included by value and not by reference (which assumes fast access to a database of Tenders).