In extragalactic spectroscopy, what is D4000? 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).
 A: A Google search turned up the following information:
D4000 is a term used by Gallazzi e. al. [1] for the 4000-Å break index of Balogh et al. [2] which they define as the ratio of the average flux densities in the spectral regions 4000–4100 Å and 3850–3950 Å.
Balogh et al. [2] found that galaxies with older stellar populations have weaker OII and Hδ lines and stronger D4000 indexes. Conversely, weaker D4000 indexes and stronger Hδ and OII lines indicate younger populations.
As it is a ratio, there are no units.



*

*Anna Gallazzi  Stéphane Charlot  Jarle Brinchmann  Simon D. M. White Christy A. Tremonti, MNRAS, 362(1), 41

*Balogh M. L. Morris S. L. Yee H. K. C. Carlberg R. G. Ellingson E., 1999, ApJ , 527, 54
A: A good technical overview is available here, and includes references for further reading. 
I'll quote one passage:

Hereafter the 4000 angstrom break is defined as the ratio between the average flux density in ${\rm erg}\,{\rm s}^{−1}\,{\rm cm}^{−2}\,{\rm Hz}^{−1}$ between 4050 and 4250 angstrom and that between 3750 and 3950 angstrom (Bruzual 1983).

As it is a ratio, it has no units. 
The drop in intensity at the blue end of the spectrum basically comes from (i) a relative lack of young, bright, blue stars in old stellar populations and (ii) increased metal absorption in stellar atmospheres in an older, more metal rich population. The particularly relevant spectral features for (ii) are Calcium H and K (3934 angstrom and 3969 angstrom), G-band (4304 angstrom), Magnesium (5175 angstrom) and Sodium (5894 angstrom).
