A public good is best described as non-rival in consumption. This means that one person's use of the good does not diminish the availability of the good for others. Public goods are typically characterized by two main features: non-excludability (it's difficult or impossible to exclude anyone from using the good) and non-rivalry (one person's use does not reduce the amount available for others). Examples of public goods include clean air, national defense, and public parks.
While they can sometimes be funded by the government, that is not a defining characteristic of public goods.