In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I with the third component written as T_z, T₃, I_z or I₃. Weak isospin is a component of the weak hypercharge, which unifies weak interactions with electromagnetic interactions. It can be understood as the eigenvalue of a charge operator.