Strangeness is a quantum number representing strange particles, a subset of hadrons containing one or more strange quarks. Each strange quark contributes 1 to the strangeness and each strange antiquark contributes -1. A particle is called strange if it has nonzero strangeness.
Strangeness is conserved in strong interactions but not in weak ones. This is useful to rule any strong interaction that would cause strangeness conservation to be violated.