Watching for particular things and recording each one you see, as a hobby. It comes from the verb 'spot' (to see or notice something, often something not easy to see), so the fun is in the catching sight of it, not in the thing itself. You almost never meet 'spotting' alone in this meaning — it comes glued to what is being watched: birdspotting, trainspotting, plane spotting, bus spotting. A spotter often keeps a list or a notebook and ticks off the ones they have seen; getting a rare one is the whole point. In Britain 'trainspotter' has become a mild insult outside the hobby — it means someone who cares far too much about small facts nobody else finds interesting — so be careful using it about a person. Note that 'spotting' has completely separate meanings elsewhere: light bleeding between periods (medical), and marks or stains appearing on something. Those are different words that happen to look the same.