The year in which your income is assessed is known as assessment year. The period for assessment year starts from 1st April and ends on 31st March of the next year.
In other words, it is the year immediately following the financial year i.e. the year in which you have earned your income.
For instance, for F.Y. 2017-18, the A.Y. would be from 1st April 2018 till 31st March 2019.