You can call it "stealing" if it was patented. And then it wouldn't be counted as his invention. Invent anything, I'm almost sure there's someone who thought of it first. You might seen it somewhere, or in your dream, or in a movie... So - to make it simple - we have patents. If you're serious about inventing things - you just file a patent. If it's granted - it's yours. If it's not yours - it's not granted ;)

I for example - invented Windows before Windows. But Microsoft made it ;) No hard feelings, I couldn't make it anyway. Seriously, I've just made a GUI on top of DOS, it displayed a movable windows with frames and some controls. 320x200, 256 colors, VGA resolution.

Now let's translate it into a lightbulb. You make a bulb with a filament that glows for a while then the filament burns. You get the idea. It took a lot of work to actually make a bulb with a filament that doesn't burn that quickly, produces enough light to be useful, at the same time being relatively cheap to manufacture. Don't forget about manufacturing process.