I would call a bootstrapped startup one that was entirely self-funded to begin with and had to operate under the bootstrapper mentality (cheap marketing, cutting costs, low employee count, etc).

Bootstrapping implies being self-funded.

To some who might have some seed funding, they might make sure the business is making money from day one.

One thing that was really attractive to me about the YC model (besides having PG as a mentor) was when there was only $10-20,000 of funding. You likely have little choice to focus on anything except making value for customers that pay. As other funding streams have come online the types of ideas have broadened.

Having no choice but to learn how to make money is how I had to learn business as a teen. I started in bootstrapping because YC didn't exist in my late teens or early 20's or I most certainly would have known it was a place for me to seek out. I had to learn to make money to pay for what I wanted to do when starting business full time in my late teens that was one valid way to make sure no one could tell me to go home because the money ran out.

In Canada, one source found that 98% of all startups across all industries are bootstrapped / self-funded. It wouldn't be surprising if this was largely true in other places too.

http://www.techvibes.com/blog/how-canadian-entrepreneurs-fin...