Knife Project
Reference
Mode
Edit Mode
Menu
Mesh ‣ Knife Project
Knife projection is a non-interactive tool where you can use objects to cookie-cut into the mesh rather than hand drawing the line.
This works by using the outlines of other selected objects in Edit Mode to cut into the mesh along the view axis, resulting geometry inside the cutters outline will be selected.
Outlines can be wire or boundary edges.
To use Knife Project, first while in Object Mode, select the “cutting object” then add to that selection with Shift-LMB the “object to be cut”. Now, enter Edit Mode and press Knife Project (found in the Toolbar).
See also
3D Viewport Alignment to adjust the projection axis.