Shopkeepers Privilege is a right or an immunity shopkeepers have under common law to detain a suspected shoplifter on store property for a reasonable period of time. To be able to establish this immunity, let us say the shopkeeper is being sued, the shopkeeper must be able to show that he/she had reason to believe that the person detained in fact committed, or attempted to commit, theft of store property.
The state of Washington has a statute. RCW 4.24.220. Careful reading of the statute adds specifics as to how the privilege may be used.