Section 2(1)(a) in The Essential Services Maintenance Act, 1968
(a)"essential service" means-(i)any postal, telegraph or telephone service;(ii)any railway service or any other transport service for the carriage of passengers or goods by land, water or air with respect to which Parliament has power to make laws;(iii)any service connected with the operation or maintenance of aerodromes, or with the operation, repair or maintenance of aircraft ;(iv)any service connected with the loading, unloading, movement or storage of goods in any port;(v)any service connected with the clearance of goods or passengers through the customs or with the prevention of smuggling;(vi)any service in any mint or security press;(vii)any service in any defence establishment of the Government of India;(viii)any service in connection with the affairs of the Union, not being a service specified in any of the foregoing sub-clauses ;(ix)any other service connected with matters with respect to which Parliament has power to make laws and which the Central Government being of opinion that strikes therein would prejudicially affect the maintenance of any public utility service, the public safety or the maintenance of supplies and services necessary for the life of the community or would result in the infliction of grave hardship on the community, may, by notification in the Official Gazette, declare to be an essential service for the purposes of this Act ;