The action or an act of eyewitnessing something; observation with one's own eyes.
Mid 17th century. From eyewitness + -ing.
That observes or describes an incident or event as an eyewitness. In early use also: †that bears witness to something (obsolete).
Early 17th century; earliest use found in William Lithgow (1582–c1645), traveller. From eyewitness + -ing.