(a)(i) doubtful asset, that is, a debt which has remained a non-performing asset of the nature specified in sub-clause (ii) for a period exceeding two years;
(ii) non-performing asset referred to in sub-clause (i) shall be the following :—
(1) term loan beyond one year, if the interest amount remains “past due” for six months or installment is overdue for more than six months;(b) loss asset, that is, a debt which has been identified as loss and considered as uncollectible but has not been written off in the accounts of the assessee.
(2) lease rental or hire purchase installment, if the rental or the installment is “past due” for six months;
(3) bill purchased or discounted, if the bill remains overdue and unpaid for six months; or
(4) any other credit facility in the nature of short term loan or advance other than those referred to in (1), (2) and (3) above, if any amount to be received in respect of such a facility remains “past due” for a period of six months;
Explanation.—For the purposes of this rule, an amount shall be deemed to be “past due” when it remains unpaid for thirty days beyond the due date.
Reference/Source: Check at Income Tax site for latest version of the Rules - link