The water cycle is a one where water turns into vapour and once again vapour condenses into water.This is a cyclic process.So,it is called as a water cycle.
water cycle is called a cycle because from where it starts it ends in there just like water first becomes vapors then vapors condenses into clouds and then clouds make rain and rain come out in form of water again therefore water returns to original form . thats why it is a cycle.
The process of starting and ending at the same point is called cycle.The water also starts in the sea as a vapour and again comes to see in the form as a rain.