An operational data system is one where you actively read and write records depending on the operations that are undertaken. A user facing product will have operations that insert, update or delete records (online transaction processing access pattern).
This is in contrast to an Analytical data system, where often you will use a read-only data set to perform analytics.
Reference
Kleppmann, Martin, and Chris Riccomini. Designing Data-Intensive Applications: The Big Ideas behind Reliable, Scalable, and Maintainable Systems. Second Edition, O’Reilly Media, 2026.