Strangeness is a quantum number assigned to the elementary particles which are conserved by the strong force. Particles are assigned a strangeness of +1, 0, or -1. Originally, strangeness was introduced to explain why some hadrons decay quickly by the strong force whilst others decay slowly by the weak force. In a strong interaction, strangeness may be conserved; i.e., the total strangeness is the same before and after the process. If strangeness is not conserved the process can only proceed by the weak force and hence only slowly.
Strangeness is currently recognized to be the third of the quark flavors.