Most people find questions like that to be annoying. Most people's working definition of 'sport' is 'whatever my culture says a sport is'. Definitions are based on tradition or authority. And most of the time, that works just fine. If you say 'sport' to someone in your culture, they will know what you mean.
But anyone with a philosophical or scientific mindset refuses to accept such definitions. We hate it when concepts are vague and fuzzy, and we hate circular logic and answers based on tradition or tacit knowledge.
Also, people probably agree less than they think. Here is a list of items. For each one, think, "Is it a sport, yes or no?"
Cross Country Running
Track and Field
I am pretty sure that your answers are different from the answers of most people reading this. How would you resolve a dispute?
One way is to identify things that are definitely sports and find their common attributes. There are probably a few things on the list that everyone would agree on. Then, for things that are not sports, analyze their attributes and see what separates them.
My office mate and I have had a couple conversations about this, and I have thought about it while driving. Here is the definition I came up with that we agreed on:
A sport is an interactive, competitive, physical activity.
Interactive means that you and other people are doing things simultaneously, and that it requires the participation of you and the others. If you can do the thing without anyone else and it is basically the same experience, then it is not interactive.
Competitive means that your goal is to win. It also implies that there is a clear ranking, especially a clear winner, and that anyone can tell who it is. If you need a special judge, or subjective judgment calls, to tell you who the best person or team is, then it was not competitive.
When a thing is both interactive and competitive, strategic thinking is required. The appropriate action to choose depends on what the opponent is doing. You have the ability to disrupt your opponent's plan or performance in some way. There are strategies and counter-strategies, offense and defense.
I could have also said "A sport is something that combines strategic thinking and physical activity." but most people do not really understand what 'strategic thinking' is. They tend to think that any careful planning or optimization is strategic thinking, but that is not the case.
Physical activity means that you will improve your performance by training your body, and that some amount of strength, dexterity, and fitness are required to do well.
Each of those three concepts are important; A thing must have all three to be a sport. If it is not interactive, then it is a contest and not a sport. If it is not competitive, than it is a performance and not a sport. If it is not physical, then it is a game and not a sport. Put another way, a sport is a game and a contest and a performance.
Other people might add the requirement that a sport requires teamwork. That is a defensible argument, but I will leave it aside for now.
When you have a definition like this, you need to test it. The most controversial part of my definition would probably be the fact that it excludes track and field events, calling them contests and not sports. I think that this is reasonable. I argue that any event where you show up, do your best at something, and hope that nobody does better than you is missing a key ingredient of sports. There is no interaction, no strategy.
My definition of 'sport' also includes billiards, and if you stretch the definition of physical activity enough, video games. I don't like that. My gut tells me that video games are definitely not sports and that billiards probably is not. I can probably get around this by fine-tuning the definition of 'physical activity' to mean something that requires more of your body than just manual dexterity.
Now, let's compare my definition to Wikipedia's:
A sport is an organized, competitive, entertaining, and skilful activity requiring commitment, strategy, and fair play, in which a winner and loser can be defined by objective means. Generally speaking, a sport is a game based in physical athleticism.
This definition adds several constraints that I feel are unnecessary. 'Entertaining', and 'skillful' are subjective terms, and in my opinion not essential to the definition. I definitely disagree with the 'requiring commitment'. If I play a game of pickup soccer once a year without any training or preparation, then I have still played a sport.
'Organized' and 'fair play' are qualifiers that I had not considered. I agree that a sport must be bound by a set of rules in order to avoid degenerating into a brawl. I guess I implicitly assumed that. Now that I think about it, my definition does include random fights, which should not be called a sport. I think 'organized' is unnecessary if 'fair play' is included because the key should be the common set of rules that the participants are using.
I also like the 'requiring physical athleticism' qualifier; it works better than 'physical activity'.
So now we have:
A sport is a competitive activity requiring strategy and fair play, based in physical athleticism, in which a winner and loser can be defined by objective means.
I think that any activity satisfying these definitions will also end up being both entertaining and skillful. Does anyone have a counterexample?