Synonyms for Witness (same or very similar meaning)
WordNet sense 1 ((law) a person who attests to the genuineness of a document or signature by adding their own signature):
attestant, attestator, attestor
WordNet sense 2 (someone who sees an event and reports what happened):
informant, witnesser
WordNet sense 3 (a close observer; someone who looks at something (such as an exhibition of some kind)):
looker, spectator, viewer, watcher
WordNet sense 4 (perceive or be contemporaneous with):
find, see
From the ODE community, based on WordNet
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