What is business cycle frequency? 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.
Business cycle frequency is a frequency that has the length of one business cycle.  Business cycle is:

economy-wide fluctuations in production, trade, and general economic activity. From a conceptual perspective, the business cycle is the upward and downward movements of levels of GDP (gross domestic product) and refers to the period of expansions and contractions in the level of economic activities (business fluctuations) around a long-term growth trend.

The length of the business cycle varies but usually, it is at least a few years.