In that most conservative of reference tools, the Oxford English Dictionary, the first definition given is a royal enclosure, i.e. a large area of land used by a small minority for outdoor recreation.
Another, more common one comes further down, noting that a park is large and public and used for recreation.
This one is from Beijing, from the university campus. Semi public then, I suppose.
This one from Shanghai. Perhaps best described as a pocket park.
This one is also a pocket park, but this is in Helsinki.
Then, the word park gets all kinds of other uses too. With any modifying word it becomes an area of land devoted to a particular activity or set of related pursuits. Note that this land may be underground too, as in this car park, probably not that far from where the previous photo was taken.
Then, back in Beijing, the Olympic Park.