Strangeness
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In particle physics, strangeness S is a quantum number needed to describe certain short-lived particles. It is defined as the number of strange anti-quarks 
minus the number of strange quarks s in a particle.
The reason for this unintuitive definition is that the concept of strangeness was defined before the existence of quarks was discovered, and for consistency with the original definition the strange quark must have strangeness -1, and the anti-strange quark must have strangeness +1.
For all the quark flavor quantum numbers (strangeness, charm, topness and bottomness) the convention is that the flavor charge and the electric charge of a quark have the same sign. With this, any flavor carried by a charged meson has the same sign as its charge.
Strangeness conservation
The strangeness was introduced originally to explain the fact that certain particles, such the kaon mesons or certain hyperons were always created in pairs. In these reactions, it was assumed that a certain property, the strangeness, was preserved.
Strangeness is conserved during the strong and the electromagnetic interaction, but not during the weak interactions. Consequently the lightest particles containing a strange quark cannot decay by the strong interaction, and their otherwise anamolously long strange lifetimes via the weak interaction led to their name. In most cases the absolute value of the variation in strangeness is 1. However, this doesn't necessary hold in second-order, weak reactions, where there are mixes of K0 and 
mesons.
See also
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