A river is a natural flowing watercourse, usually a freshwater stream, flowing on the Earth's land surface or inside caves towards another waterbody at a lower elevation, such as an ocean, sea, bay, lake, wetland, or another river. In some cases, a river flows into the ground or becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to by names such as creek, brook, and rivulet. There are no official definitions for these various generic terms for a watercourse as applied to geographic features, although in some countries or communities, a stream is customarily referred to by one of these names as determined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and Northeast England, and "beck" in Northern England. Sometimes a river is defined as being larger than a creek, but not always; the language is vague.

नदी म्हणजे मोठ्या भूप्रदेशावरून एका दिशेकडून दुसरीकडे वाहत जाणारा नैसर्गिक रुंद थंड पाण्याचा प्रवाह. नदीचा उगम हा तलाव, मोठा झरा, अनेक छोटे झरे एकत्रित येऊन किंवा बर्फाच्छादित पर्वतापासून होतो. रचनेच्या दृष्टीने त्या भूप्रदेशातून जल निःसारण करणाऱ्या सर्व जल प्रवाहांची मिळून नदीप्रणाली होते आणि त्या प्रदेशाला त्या नदीचे खोरे म्हणतात.