Witness
noun
An evidence person is a witness, because witnesses are people who provide evidence (you know, with their mouths or something).
しょうにん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
証人はいません。
There are no witnesses.
私も証人の一人です。
I am also one of the witnesses.
彼はエホバの証人です。
He is a Jehovah's Witness.