What is OLAP?
OLAP describes systems and queries built for analysis: scanning and summarising large amounts of historical data across many dimensions.
OLAP explained
Typical OLAP questions are 'total revenue by product and month for three years'. Data warehouses and analytics engines are tuned for this, often storing data by column so that aggregations over a few columns are fast.
OLAP queries read a lot and write little, which is the opposite of the workload of a transactional system.
Example
Asking for average order value by country and quarter over five years is an OLAP-style query.
How it differs from similar terms
OLTP systems handle many small reads and writes, such as recording each order as it happens.