Final answer:
Aggregating event records into a single summary record is known as a Physical level implementation compromise, which reduces data size for optimization of database performance.
Step-by-step explanation:
Aggregating a group of event records into a single summary record once historical detail is no longer needed can be described as a form of data management strategy that falls under the category of a Physical level implementation compromise. This process involves summarizing or data aggregation to optimize database performance or to conform to data retention policies. In the context of database management systems (DBMS), this operation is usually performed as a means of database optimization and could involve creating a summary table or rollup, which reduces the amount of detail stored and thus can speed up query times on large datasets.