Dear Sujal,
Technical definition can be obtained from the Industrial Disputes Act. Grossly, retrenchment means any termination that is not a disciplinary action imposed on a worker, voluntary retirement scheme (VRS), retirement, superannuation, ending of a fixed-term contract, or termination of service due to continued ill health.
Lay off means failure, refusal, or inability of the employer due to a shortage of raw material, coal, power, breakdown of machinery, etc., to provide employment. It is of a temporary nature.
Termination means dismissal, retrenchment, completion of a contract, non-renewal of a contract, etc. Termination is not always negative. All these activities are termed as termination.