The first scheduled commercial airline flight across the bay from St. Petersburg to Tampa, Florida, took 23 minutes on the morning of 1 January, 1914. Tony Jannus piloted a Benoist flying boat with Abram C. Pheil, the former Mayor of St. Petersburg, as his passenger. This marked the dawn of regular passenger air travel.
There are dozens of opinions about what constitutes a commercial flight, but the only one that is important is what the FAA says is a commercial flight. And even that can be a gray area. The FAA has no specific rule that defines “Commercial Flight”, but 14CFR §61.133 – “Commercial pilot privileges and limitations” is the closest thing you will find to a definition: “A person who holds a commercial pilot certificate may act as pilot in command of an aircraft carrying persons or property for compensation or hire.” (Yes, I paraphrase).
Carrying persons leaves out our small UAS aircraft. Carrying property probably doesn’t concern us either unless you fly for Amazon or Google. It’s obvious that if someone pays you to fly to take real estate photographs, for example, then you have been hired. Which leaves “for compensation”. Compensation is the most vague term in the rules, and it’s not defined. So is generally up to the enforcing official to define Compensation. According to the FAA if anyone materially benefits from the flight, anytime, it is a commercial operation. Even posting a hobby flight video on YouTube was getting warning letters from one zealous FAA inspector because Google would attach advertising to the video and could earn money from it. Fortunately the head of the FAA enforcement department saw the stupidity of that and issued a Policy Notice (PDF) that told the field inspectors to lay off.
Profit or lack of it has no bearing on whether the flight is commercial or not. If you take photos or video from your drone for a charity, then good for you. But if the charity uses one of those photos in promotional material or an ad, it’s a commercial flight.