What is the purpose of the `lookup` stage in the aggregation pipeline?

The lookup stage in the aggregation pipeline is specifically designed to facilitate a left outer join between two collections within a MongoDB database. This stage allows you to combine documents from the input collection with corresponding documents from a specified foreign collection based on a defined condition.

By using the lookup stage, you can enrich your data by incorporating related information from another collection. For instance, if you have two collections, one containing orders and another containing customers, you could use lookup to retrieve customer details for each order, thereby creating a more comprehensive dataset for analysis or reporting.

This capability to join data from separate collections makes it a powerful tool for data transformation and analysis, enabling users to create complex data queries that reflect relationships across different sets of data. The other options do not accurately represent the function of the lookup stage; they pertain to different operations within the aggregation framework.