The Bottom-up testing is a testing approach which follows integration testing where the lowest level components are tested first, after that the higher level components are tested. The process is repeated until the testing of the top-level component.

Bottom-up testing is an incremental approach to integration testing where the lowest-level components are tested first, and then progressively higher-level components are tested in subsequent steps. The testing process moves from the individual modules or units towards the complete system.

In bottom-up testing, the focus is on testing individual components or units in isolation to ensure that they function correctly. Once the lower-level components are verified and validated, they are integrated to form higher-level components, and the testing process continues upward until the entire system is tested.

This approach allows for early detection and resolution of issues at the component level and ensures that the integration of components is smooth and error-free. Bottom-up testing is often contrasted with top-down testing, where testing starts from the highest-level modules and progresses downwards. Both approaches have their advantages and disadvantages, and the choice between them depends on the specific requirements and characteristics of the software project.