One who smokes very infrequently, such as after a meal, when drinking, or just socially.
I fear I was one of these people. For years I prided myself on never having smoked a cigarette... until I noticed I basically had anyway, given that smoking solid usually involves tobacco. Plus, I hung around smoky places, many of my friends were smokers and so I was inhaling huge clouds of nicotine on a regular basis.
So I tried one. And, unlike the first ever cigarette of lore, I didn't gag, retch, cough or shudder. After all, I've been breathing this crap in by the simple act of having a social life for years.
The most cigarettes I've ever smoked in a day is a mere three, and only because my friend kept offering them as thanks for buying her the pack (she's old enough, but hates getting carded by the woman in the supermarket).
From this unique perspective, I believe I've figured out a reason why people smoke. It's something to do! You're not stuck with two empty hands to fidget, you have a cigarette to make you busy. Bored and waiting for someone? Have a smoke! Lecture break? Light one up! It's like having a drink in your hand at a party, it gives you something to fiddle with.
I'm not alone as a casual smoker. Another friend's sister is a dedicated casual smoker, who goes through a pack maybe every ten days. I'm nowhere near this. I haven't smoked in well over a week either, despite having a pack in my pocket. This is partially because I simply don't crave one, and partially because I can't be bothered explaining this to everyone who knows me as a non-smoker.
From all this I conclude that it is possible to smoke without being addicted, and therefore understand how many don't see anything wrong with a cigarette every now and again. After all, alcohol gives you cancer too.