There are words in our tradition which are not translatable into English or other European languages. Sukh cannot be translated as happiness. The famous Shloka of the Bhagavad Gita: Sukh-dukhe samekritva, labhalabho jai jayo; cannot be translated as ‘Be unperturbed in happy and unhappy times’. Sukh is a subtle state of mind with inner joy accompanied with peaceful mental state. Sukh is a state when the mind is not perturbed with troubled emotions and there is a sense of satisfaction or santosha. The mind is in harmony with its surroundings. On the contrary, happiness is a state of elevated emotions, which is related to the rajas state of mind.
Etymology of the word sukh is very interesting. su means good and kha means aperture and it refers to a good axel hole. In Rig Veda, the word sukh denotes ‘running swiftly’, obviously related to a good axel whole and running of a wagon. Probably this explains well that sukh is not merely happiness but a smooth running of life with a sense of satisfaction.