What's the difference (if any) between a co-founder and a founding engineer (or founding team member)?

It is completely arbitrary and negotiable with the company you're joining. Perhaps the question you should ask of yourself is, why does it matter? Is the financial reward not enough? Do you feel the title would be beneficial in your future career? Do you feel that your boss doesn't respect you enough? The answers may indicate that the title is not important, or perhaps there is another more serious issue between you and the founders.

The title of co-founder isn't set in stone to any time or round concerns. Like any other title, it's established by the hiring company and individual they are hiring. Like any title, it's up to negotiation between you and the other members of the venture. There are standards here, and it sounds like your current title is well within the norm of that.

The difference between co-founder and founding member in real-world practice is subtle but critical: 1. A "Co-Founder" is an actual Title usually reserved for those who started the company, who bring their own capital and/or angel funding AND (most important) own a major part of the start-up's "EQUITY". They can use it as Official designation to differentiate themselves in the market from other senior management and even have it in their business cards. They are usually part of the Board of Directors (until kicked out by an unscrupulous VC or other co-founders / partners). 2. Any early employee (usually those who join first few days of start-up) can legitimately call themselves "Founding Member" or "Founding Team Member" (though founding member is preferred term nowadays). Founding members don't put "founding member" as a title in their business cards or official designation as it is more "descriptive". You will not find "VP - Tech and Founding Member" but only "VP - Tech" and the word "founding member" used in their bios/profiles. Founding members are given ESOPs and stock grants by the Co-Founders and Board.

Founder vs Co-Founder   Founder is a word that we are familiar with and understand it as the person or individual who establishes a venture. It is a term that connotes pride and prestige as well as creativity on the part of the person who starts the venture. However, there is a term co-founder that clearly indicates that there is not a single person who has started the venture. This article attempts to find out if there are differences between a founder and co-founder or is it merely a word to indicate more than 2 people involved in starting a venture. Founder If there is a single person who, not only has the idea, but also the resources to turn that idea into a successful venture, that person is termed as the founder of a company or a venture. History is replete with business empires and ventures that were created by the vision and fearlessness of single individuals who dared to turn their dreams into realities. A founder is a person who creates something out of nothing. Most big and successful companies around the globe started out with humble beginnings because of the courage and vision of their founders. Co-Founder Many a times, the establishment of a business or a venture is the brainchild of 2 or more individuals who share not just this idea but their resources and expertise to establish a business. In such a case, all these members of the group are referred to as co-founders of the establishment. As the word indicates, a co-founder is a person who founds an entity in conjunction with another individual. There may be several co-founders in a company, and the recent spat between actor Ashton Kutcher and the co-founder of Apple, Steve Wozniak has brought out this word in the news once again. While millions believe it was just Steve Jobs who was the founder of Apple, it turns out that he established the business in association with Wozniak. What is the difference between Founder and Co-Founder? Founder is the individual who has the idea or vision to start a venture from scratch. Co-founder is a term that is used to refer to the members of the group that takes the initiative to set up a venture. Founder is not a term that can be used in a hierarchical manner to indicate the superiority of a single individual in setting up a venture. This is why all those who support an idea and pool their resources to start a venture are called co-founders. Everyone who comes after the stage of ownership of a venture is an employee and not a founder.

never heard of founding engineer.  I use "risk" as metric to asses if you are worth the title of co-founder. I consider co-founders those who in addition to conceiving the idea and building the first mvp, 1) took the risk of starting a business with their own savings, 2) worked on it for months without a salary. If you are the employee #1 and you are given a salary/equity package, then you are taking a much lower risk. Equity is generally how first employees are compensated for that risk.