Etymology From Middle English hyghnesse; equivalent to high + -ness.
1
Usually The state of being high.
'It is as high as heaven, what canst thou do?' It is as the highnesses of heaven: take all the heights and elevations, all the spheres and altitudes of heaven, and try if thou canst reach them with thy short arm; yea, climb up the highest storeys, the loftiest pinnacles, […]1672, George Swinnock, The Incomparableness of God:
2
Usually A title of respect when referring to a prince or princess.
Etymology Calque of Italian altezza, from Late Latin altitia, from altus + -itia, equivalent to high + -ness. See highness for formation of the English term.
1
The style of a prince, princess or someone of equivalent rank.