Extract, Transform, Load — a data integration pattern for moving data from source systems to a destination, applying transformations along the way.
ETL is the standard approach for building data pipelines. Extract: pull data from one or more source systems. Transform: clean, standardize, and reshape the data. Load: write the transformed data to the destination (a database, data warehouse, or file).
Modern pipelines sometimes use ELT (Extract, Load, Transform), loading raw data first and transforming it in place using the processing power of the destination system.