Great introductory article on #matroid s:

Neel, David L., and Nancy Ann Neudauer. 2009. “Matroids You Have Known.” Mathematics Magazine 82 (1): 26–41. https://doi.org/10.1080/0025570X.2009.11953589.

#Matroid s are a specific kind of sets that contain other sets, but for any set they contain, they also need to contain its subsets. For more look here:

https://en.wikipedia.org/wiki/Matroid

Looking oddly specific, they're in fact an interesting structure which pops out in the study of many combinatorial subjects like graph theory, and, as I just learned, #homology!