Google Cloud's fully managed relational database that scales horizontally worldwide with strong consistency and ACID transactions across regions. In a multi-region configuration it offers a 99.999% SLA.
In CDL it is distinguished from Cloud SQL: Cloud SQL is for moderate-scale relational workloads in one region; Spanner is for global systems with huge transaction volumes and strong consistency. Question: a core banking system or global inventory with no downtime → Spanner.
Cloud Spanner is a concept that can show up on the exam. Check the full guide to see it in context.