Conceptualized, but also pondered upon, and considered worthy of further effort. A conceptualization can be done as fast as a single second, but it can take ages to weigh it against alternatives.
You can apply the "conceptualization" criterion retroactively to any successful project. They all started as an idea that someone noodled around with for long enough to decide to pursue it further. You can draw the line where that initial noodling ended and serious work began pretty much anywhere you like.
