A founder is a person who's in at the beginning. But beginnings are slippery things. For me there are three characteristics I look at: early contribution, risk, and ownership.

The most obvious founder is the solo founder who worked a long time on their own, took most of the risk, and owns the whole company. When they eventually hire employees, those people come into an established structure and are just there for a paycheck. Those employees didn't make an early contribution, they aren't taking much risk, and don't have ownership, so they're clearly not founders.

It gets fuzzier when you have more people, especially ones who didn't all start at the same time. If two people bump along for a while and add a third, can one call that third person a founder? Often I'd say yes when that third person made key early contributions, took a lot of risk, and was rewarded with a significant percentage of the company. They weren't there at day 1, but they still joined before the company had significant investment or traction. Without them, the company really wouldn't be the success it ended up being.

An example from my own life: I was one of 6 people who were there at the beginning of Sidereel. I spend a couple of months with them defining the product and writing a lot of the early code. But then I left to do other things. Was I a founder? I say no: I was there from the start, so I made an early contribution, but by leaving I passed on the risk and the ownership.