Symmetry Mode Toggle
From Nevercenter 3D Modeling Wiki
Default Shortcut: Alt + N
Menu Path: Modify > Mirroring > Symmetry Mode Toggle
You can use this command to enable or disable symmetry on your model. If symmetry has not yet been calculated (see Calculate Symmetry), it will be calculated before being enabled. You may wish to temporarily disable symmetry in order to add asymmetrical details to your model.