Definition of "essentially"

A doorknob of whatever roundish shape is effectively a continuum of levers, with the axis of the latching mechanism—known as the spindle—being the fulcrum about which the turning takes place. Applying a force tangential to the knob is essentially equivalent to applying one perpendicular to a radial line defining the lever.

2012 March, Henry Petroski, “Opening Doors”, in American Scientist‎, volume 100, number 2, pages 112–3