Modify panel > Make a selection. > Modifier List > Object-Space Modifiers > Mirror
Make a selection. > Modifiers menu > Parametric Deformers > Mirror
The Mirror modifier provides a parametric method of mirroring an object or a sub-object selection. You can apply the Mirror modifier to any type of geometry, and you can animate the mirror effect by animating the modifier's gizmo.

Mirror Center—Represents the axis of the mirror effect. You can move, rotate or scale the gizmo to affect the mirroring. You can animate the gizmo transforms, which you can't do with the toolbar Mirror tool.
For more information on the stack display, see Modifier Stack.