Cardinal characteristics of continuum are various cardinals which are typically between $$\aleph_1$$ and $$2^{\aleph_0}$$ and their definition often has a combinatorial flavor. Some examples are:
• The cardinal $$\mathfrak p$$ - the smallest cardinality of subsystem of $$[\omega]^\omega$$ with strong finite intersection property and no pseudointersection.
• Various cardinals related to $$(\omega^\omega,\le^*)$$ such as the bounding number $$\mathfrak b$$ (=the smallest cardinality of an unbounded subset) or the dominating number $$\mathfrak d$$ (=the smallest cardinality of dominating subset).
The tag also encompasses analogues at larger cardinals, such as the bounding number $$\mathfrak b(\kappa)$$ defined in terms of families of functions from $$\kappa$$ to $$\kappa$$, and cardinal invariants of certain structures (such as topological spaces or Boolean algebras).