There must be sufficient circumstantial evidence in order to indict someone of a crime, which would mean that sufficient evidence exists to warrant having a criminal trial to try the defendant. Eyewitness testimony is considered evidence, e.g. a police officer witnesses you trespassing. It is a low threshold of proof -- probable cause to believe that a crime has been committed and that the defendant committed the crime.
The definition of evidence is any proof that is relevant to a case that (1) helps determine whether something is more true or less true, and (2) is reliable and can be authenticated.