Can 斋戒 vs. 禁食 which one is used for "Fasting for ramadan"
禁食 - Forbidden to eat or drink. i.e. fasting.
斋戒 - This word can have a different meaning according to different religions. For example:  


Islam - no food or drinks at all  
Christianity - no meat from warmblooded (i.e. fish / shrimps are allowed)  
Chinese Buddhism - no meat / alcohol and certain types of vegetable  



Therefore, it depends on the situation where you will use the words.