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; in English the language is vague compared to some langauges like French, where a fleuve flows into the sea and a rivière is a tributary of another rivière or fleuve.

ಹೆಚ್ಚು ಪ್ರಮಾಣದ ಹರಿಯುವ ನೀರಿನ ಒಂದು ಸಮೂಹಕ್ಕೆ ನದಿ ಎನ್ನಬಹುದು. ನದಿಗಳು ಸರೋವರದಲ್ಲಿ, ಬಗ್ಗೆ ಅಥವಾ ಊಟೆ(spring)ಯಲ್ಲಿ ಅಥವಾ ಕೆಲವು ಚಿಕ್ಕ ಕೊಳಗಳ ಸಮ್ಮಿಲನದಲ್ಲಿ ಉದ್ಭವಿಸುತ್ತವೆ. ಉದ್ಭವದಿಂದ ಕೆಳಕ್ಕೆ ಹರಿದು ಮಹಾಸಾಗರಗಳಲ್ಲಿ ವಿಲೀನವಾಗುತ್ತವೆ. ಪುರಾತನ ಕಾಲದಿಂದಲೂ ನದಿಗಳು ಮಾನವ ನಾಗರೀಕತೆಗಳಿಗೆ ಅತ್ಯಂತ ಮುಖ್ಯವಾಗಿವೆ. ಪ್ರಪಂಚದ ಅನೇಕ ಪ್ರಮುಖ ನಗರಗಳು ನದೀತಟದಲ್ಲಿ ಸ್ಥಿತವಾಗಿವೆ.