Example: You watch the front door of an office building. From seeing who walks in, you ascertain with 99% probability that companies X and Y are working on some kind of big deal. If you're just a random person with no relationship to those companies, you can trade on this information.
(Not legal advice! Don't do this unless you are sure it is OK! Which I am not!)
> Illegal insider trading refers generally to buying or selling a security, in breach of a fiduciary duty or other relationship of trust and confidence, on the basis of material, nonpublic information about the security.
If you have no fiduciary duty or other relationship of trust and confidence, it’s not insider trading. So to my understanding, if you, say, overheard two execs chatting about this in an airport lounge you’d be free to trade on it.
Note: this only applies to the US! Different countries may define things differently.
[1]: https://www.investor.gov/introduction-investing/investing-ba...