Here, visualized, the results of a poll that are as interesting as they are unscientific. Click expand and commence arguing over what makes something a sport as opposed to a game or an activity.
The results come from Reddit user "e8odie," who had self-selected members of three different subreddits (/r/SampleSize, /r/sports, and /r/CFBofftopic) answer yes/no to whether they considered 53 different "fringe" games as sports. He got 460 total respondents, and their results are above.
It's contentious, but it seems something can be considered a sport if it a) requires athleticism or hand/eye coordination, b) clearly identifies a winner, preferably without the use of judges, and c) meets some nebulous criteria of being "mainstream" enough.
One thing's for sure: croquet's legitimacy is the great debate of our time.