"Startup" usually means a company that fulfills a few of the below:
- something the founders do full-time
- less than 3 years old
- in a bootstrap phase or burning VC money
- in search of a business model (not yet profitable)
- has the potential to growA profitable startup with a great business model which doesn't need to give away ownership for VC money is the perfect startup.
Any business that's new counts as a startup for me.
People start and build businesses while employed all of the time. Some of those businesses require that the owners devote their full energies to them at a certain milestone.
Others do not -- plenty of software and consulting roles do not require 100% commitment, particularly if you have a flexible employer and can bootstrap. Many college professors have side gigs, for example.
Other businesses can start slow/small, but you reach a point where you need to choose between the full time gig and your startup. There are a few clear examples of this phenomenon in this thread.
Starting a business has been done in many ways. Having been in business for 15 years I have gotten to meet a lot of other small business owners and there is as much variety in how established businesses came to be.
Having this delusion of its only a "real" startup if you do it this way is hogwash. So many great businesses were built on hard work, sacrifice, long hours, juggling more than one thing at once, until the new business could stand on it's own two feet.
Knowing when, and what your new business needs (more time, stepping away from other obligations) is a critical skill. Simply committing all of your time to one thing without a wider business, or varied experience can in some cases be a detriment.