This is the 14472nd most frequent Russian word.
очевидец
Eyewitness.
The word 'очевидец' is used here as a subject referring to a person who witnessed an important event.
Он был очевидцем важного события.
He was a witness to an important event.
In this sentence, 'очевидцы' is used as the plural form of the word, referring to multiple people who saw something happen.
Очевидцы описали происходящее очень подробно.
The eyewitnesses described the event in great detail.
Here, 'очевидец' plays the role of a predicate nominative, describing the subject's role as someone who witnessed something.