The -oid suffix means "similar to" or "resembling."

For example: a humanoid is something resembling a human, but not a human.

So, traditionally, a factoid is something resembling, but not, a fact.

However, it has been misused for so long, it's now considered by many ok to use "factoid" to mean a trivial, related fact.

There is no "the" dictionary. And in most respected dictionaries, it isn't the first definition.

https://www.merriam-webster.com/dictionary/factoid

https://en.oxforddictionaries.com/definition/factoid

Some dictionaries order senses by age and others by usage (which may be somehow measured or may just be up to the instincts of the editors), so this isn't a useful metric without knowing the standard of the dictionary we are talking about.

Which dictionary do you mean? Do you mean that highly inaccurate one that you get when typing a word into Google.com?