In a generic sense, the terms " lay off " and " termination " as used in the realm of employment across the world means the same indicating the termination of employment of any number of persons particularly due to lack of work at the instance of the employer.
But, according to the Industrial Disputes Act,1947, it is a temporary cessation of employment of any number of workmen employed in an industrial establishment because of the failure, refusal or inability of the employer due to certain specific reasons mentioned u/s 2(kkk) of the Act. The distinction is that there is the subsistence of the contract of employment during the entire period of lay off and payment of lay off compensation subject to certain conditions.
In the scenario described in the initial post, it is termination of employment amounting to retrenchment in respect of those who did not resign their jobs..