to different people, it can’t be defined. If for example someone claims to have a rash from playing in the sand and another person recognizes it as a disease not at all related to the sand, that doesn’t make them both true, especially when one of them is a dermatologist who knows her stuff and the other is a psychic.
What a person thinks may be irrelevant in establishing truth. It is one of the reasons that intellectual discussions regard informed opinion as more useful than mere opinion. Opinions come from the mind, and near empty or garbage-filled minds can muddy the waters at best. Informed opinions, otoh, at least lend some credibility. That’s one reason why “eye witness” testimony alone is considered insufficient in determining what actually happened in any given event.

Nailing Jello to a tree is silly to watch, sillier to do.