Personally a sport, for me, must involve some kind of physical dexterity. Thus darts easily falls into that category because it's largely based around fine motor control.
'Physical' effects are different. *Everything* is physically taxing in some form or another, that doesn't make it sport. Work is tiring, having sex is tiring, listening to music for hours on end has physical effects, but it's not *sport*. There is no element of physical dexterity or motor skill.
Hugely, hugely different distinction - and not one that you can weasel out of by saying that it's tiring or has physical effects on the body. Now as I said before, I'm actually on the fence about poker (and chess actually) because I believe they are intensely challenging mentally (often more so than many physical sports), but that element of physically based skill is basically absent.
gc.