Over the past several years, perfusion has become an increasingly attractive process for development in the biopharmaceutical industry. It is seen as an opportunity to improve overall productivity, and unlike fed-batch culture, perfusion can be integrated into continuous bioprocessing workflows for constant product production and purification. Furthermore, perfusion processes can be extended for longer run times, compounding these increased productivity benefits. To fully evaluate whether a perfusion application will work in a production process, it is critical to have standardized terminology. This white paper explains key terms that are essential for understanding the process.