Why alkenes called olefins?
It was derived from the French term "gaz oléfiant".
It was derived from the original French "gaz oléfiant" which means "oil forming gas". Olefin was the original term, "alkene" was coined later to give a more systematic naming system (-ane, -ene and -yne, relating to aliphatic hydrocarbons with single bonds only, one or more double bonds, or one or more triple bonds, respectively).
In industry, however, (as opposed to schools and colleges) the term "olefin" is still the more commonly used term - e.g. the chemical company Ineos Olefins and Polymers.