I'll probably get some flak for this, but it's fine. To me, a competition can only be considered a 'sport' ONLY IF your opponent(s) can DIRECTLY affect your results. Thus, tennis passes but not golf.
For me, golf isn't a sport. It's a 'leaderboard' competition. It's highly skilled, yes, but you're really playing by yourself. I understand that the mental side can be affected by other competitors, but those competitors CAN'T physically affect your play, make you play bad, nor shut you out altogether. And those mental aspects are already in other competitions that I deem as 'sports' too. Golf is just marketed very well. I mean, what makes the world's best golfer greater when compared to the world's best carpenter/darts/archery/etc. (skill-wise on a leaderboard i.e. 'best in the world')? They all require incredible skills that most cannot hope to compare to, but none of the participants are affected by other competitors. This is not true for what I deem as 'sports' such as baskeball/baseball/football/soccer/tennis/boxing ('blood' sport)/etc. These sports not only require you to be highly skilled, but even at your best, your results are highly dependent on what your opponents 'allow' you to do.