Lookup transformation is used to look up data in a flat file, relational table, view or synonym. Lookup is a passive/active transformation and can be used in both connected/unconnected modes. From informatica version 9 onwards lookup is an active transformation. The lookup transformation can return a single row or multiple rows.
You can import the definition of lookup from any flat file or relational database or even from a source qualifier. The integration service queries the lookup source based on the ports, lookup condition and returns the result to other transformations or target in the mapping.