I am reading a paper by Michal Andrle, Jan Brůha, Serhat Solmaz (On the sources of business cycles: implications for DSGE models). The authors use data at a quarterly frequency for their empirical analysis. But they also have this statement in their paper, 'Our empirical approach boils down to a multi-country dynamic principal component analysis of data at business cycle frequencies'.
What does the term 'business cycle frequency' mean? And what does it mean to analyze data at business cycle frequency? I have yet found a simple definition for it.