Public goods are goods that can be consumed simultaneously by a large number of people without the consumption by one imposing an opportunity cost on others. 

 The characteristics of a public good: 

• Non excludability i.e., the citizens can enjoy its benefits at no explicit financial cost.  
• Non rival consumption i.e., the marginal cost of supplying this public good to an extra citizen is zero.  

• Non-Rejectable i.e., collective supply for all citizens means that it cannot be rejected. 

Source: NCERT XII Macro Economics