The term "second nature" is often used to describe something that has become natural or instinctive to a person. However, there are several other phrases that can be used to convey a similar meaning. One such phrase is "habitual", which suggests that an action or behavior has become a routine part of someone's life. Another synonym for "second nature" is "ingrained", which implies that a habit or behavior has become deeply rooted in a person's psyche. "Instinctual" can also be used to describe something that comes naturally to a person without conscious effort. Overall, these synonyms help to convey the idea that a behavior or action has been repeated so often that it has become a natural part of who a person is.