What term is defined as the substitution of a pleasant word or phase for a more direct one?

The term you're looking for is "euphemism." A euphemism is a figure of speech that replaces a harsh, blunt, or unpleasant term with a more gentle or vague expression. This is often done to soften the impact of the message or to adhere to social norms regarding sensitivity. For example, saying "passed away" instead of "died" is a common euphemism.