The discussion centers on the confusion between thermal and mechanical efficiency in Stirling heat engines. Mechanical efficiency is described as the ratio of work done by the gas to the work output of the engine, while thermal efficiency relates to the thermodynamic performance of the engine. Participants emphasize the importance of clear definitions from sources discussing these terms. There is a consensus that mechanical efficiency should not be used interchangeably with thermal efficiency. Understanding these distinctions is crucial for accurate analysis of heat engine performance.