Customize menu > Grid and Snap Settings > Snaps tab
Snap Override lets you supercede all the currently selected snap types and temporarily use only one, or none, of the snap types currently selected on the Grid and Snap Settings dialog. For example, you might be creating a spline while snapping to Grid Points, but then need to snap one of its vertices to the midpoint of an object.
Procedure
To use Snap Override:
Hold down SHIFT and right-click in a viewport.
Select one of the snap types from the menus in the Snap quadrant to make it the only active snap type.
If the Grid and Snap dialog is displayed, the "Override OFF" label displays the selected snap type. When you complete the mouse action, "Override OFF" is again displayed in the dialog, and the previously active snap types are active again.
To use Snap Override during a drag operation, do one of the following:
Keep the left button pressed, select from the menu with the right button, and then continue the drag operation.
Release the left mouse button after the menu appears, select the snap you want with a left-click, move the mouse with no buttons pressed (the geometry remains locked to the mouse), and then click to complete the operation.
Interface
In addition to the available snap types, the snap quadrant contains these items:
Options Sub-menu
Transform Constraints: Turn this off to ignore the current transform constraints. For example, if you're moving a vertex with Restrict to XY Plane on and want to snap the vertex to a point removed on the Z-axis, turn this off. Default=on.
Snap To Frozen: Turn this on to enable snap to frozen objects. Default=off.
Last: Displays the last snap type you chose, letting you easily reselect that snap type and use it again. (This item is disabled if no previous snap type was chosen.)
None: Turns off all snap types for the next mouse action. (This item is disabled if the Snap Toggle is off.)