A tectonic window is an area where older rocks, typically part of a lower structural unit, are exposed through erosion or tectonic processes, surrounded by younger, overlying rocks. Tectonic windows are significant in structural geology for understanding the history of tectonic uplift, erosion, and the exposure of deep crustal rocks.
Reference: Twiss, R. J., & Moores, E. M. (2007). “Structural Geology.” W. H. Freeman.