A strange particle is a particle, specifically a hadron, that exhibits anomalous characteristics: it decays with a mean lifetime typical of the weak interaction (), but scatters with a cross section typical of the strong interaction (). They often decay in a pair of charged particles, to the point that their first recorded name for them is V particles, from the V-shaped tracks their decay products made in cloud chambers from cosmic rays.
The strangeness, as it is called, is due to the presence of the aptly named strange quark present in these hadrons. Strange particles are defined by having a nonzero strangeness quantum number. The best known strange particle is the kaon, a ~500 MeV meson. Several more strange particles have been discovered around the same time as the kaon, all of which were Spin 1/2 hadrons heavier than the neutron: they are collectively known as hyperons.
The requirement of a nonzero strangeness number leads to the curious case of the phi particle (quarks: ) not being strange despite being entirely made of strange quarks, since the two cancel each other's strangeness.