In relation to astronomy what is D4000? In my work I understand it is a good indicator of a stars age and I believe it is a spectral region, but I am having a hard time even finding resources to help me better understand it. My specific questions are this: what is D4000? what units is it measured in? how is it used to determine stellar population age? Alternatively, what are some resources or recommended literature for getting a better understanding of it?
For some additional context, I am reading Van Der Wel, et al. (2016).