what is poverty line ?

 Poverty line is defined as the income level which is just sufficient to buy the basic ,minimum quantity of food required.

it is an imaginary line.

A person is considered poor if his income or consumption level falls below a given "minimum" level necessary to fulfil basic needs

The estimated minimum level of income needed to secure the necessities of life.

In India, calorie based norm is used to define poverty. The minimum average calorie requirement is determined by the planning commission at 2435 calories and 2045 calories respectively for the rural households and urban households. Based on the calorie requirements, in 1999-2000 poverty line was fixed at Rs 328 per person per month for rural households and Rs 454 per person per month for urban households.