Basically a flood is a when the water level in an area rises where there was normally little or none before. Floods can be dramatic and quick or slow and creeping.
Floods are usually natural phenomena common in many places around the world where either there is river nearby or the local weather can dump large amounts of rain.
The word 'Flood' comes from Old English, Flod, or Pleu, of an Indo-European language root.
Other uses of the word flood
There are many other uses of the word flood, which generally means a sudden increase in the amount of just about anything. Flood with light, flood the market with cheap goods and so on.