Emotivism / Expressivism
Emotivism is the noncognitivist metaethical theory according to which all moral statements are mere expressions of our attitudes.
An emotivist would analyse a moral statement such as “It’s morally good to give money to charity” as equivalent to “Hurray for giving money to charity!” Similarly, according to emotivism the statement “It’s wrong to take innocent lives” means merely “Boo to taking innocent lives!”.
For this reason, emotivism is sometimes referred to as “Boo-Hurray theory”.
Emotivism is a noncognitivist theory because it holds that moral statements are neither true nor false, and so cannot be known. “Hurray for giving money to charity!” is no more true or false than “Hurray for marshmallows”; it is just an expression of one’s feelings.