Object and Edit Modes
Mirroring an Object or Mesh selection will create a reversed version of the selection. The position of the mirrored version of the selection is determined by the Pivot Point. A common use of mirroring is to model half an object, duplicate it and then use the mirror transform to create a reversed version to complete the model.
Mirrored duplicates can also be created with a Mirror Modifier.
To mirror a selection along a particular global axis, press: Ctrl-M, followed by X, Y or Z. The image Mirroring a Selection shows the results of this action after a mesh element has been duplicated.
In Mesh mode, you can mirror the selection on the currently selected Transform Orientations by pressing the appropriate axis key a second time. For example, if the Transform Orientation is set to Normal, pressing: Ctrl-M, followed by X and then X again will mirror the selection along the X axis of the Normal Orientation.
You can alternatively hold the MMB to interactively mirror the object by moving the mouse in the direction of the mirror axis.