What is definition of Zakaat?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Zakah (alms) linguistically means an increase, excess or build up, and in Islam it means to pay a particular amount which has to be paid on a given number or a given quantity of some particular things, under certain circumstances.  This amount has to be paid when the money or other Zakatable things reach the minimum amount on which its Zakah has to be paid and when a complete lunar year passes, except in the case of fruits and crops, it is not a requirement that a whole year should pass after harvesting.

Allaah knows best.