The short answer is that they are not interchangeable terms. Think of non-alcoholic fatty liver disease as an umbrella term (NAFLD) and non alcoholic steatohepatitis (NASH) falls under that. So, steatohepatitis is a type of NAFLD. Fat will first accumulate in the liver, this can then progress to steatohepatitis which means fat accumulation and inflammation. Hope this was helpful!