Well, let us take the help of Wikipedia:
Illusion: An illusion is a distortion of the senses, which can reveal how the human brain normally organizes and interprets sensory stimulation. Although illusions distort our perception of reality, they are generally shared by most people.
Hallucination: A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception.
Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted stimulus (i.e., a real perception) is given some additional significance. Many hallucinations happen also during sleep
paralyses.
Hallucinations can occur in any sensory modality - visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive.
"Does reality need to be corroborated by science or your fellow human to be considered "reality"? If so, why?": Yes, what we perceive as reality has to be checked (normally the brain does it without our realizing it). For example, a mirage is an illusion. We see it so very often in summers of New Delhi as if there is water on the road. Our brain checks (without our realizing) whether there are clouds in the sky, has a drain or a pipe is broken somewhere, has a municipal cleaning van has showered water on the road (then, no part of the road should be dry, not just the patch in distance).
April 20, 2017. 43 degree centigrade (That is the Indian President's Official Residence).
Mirage seen in New Delhi - Google Search