Would define means to describe a meaning only in an imagined situation. Would is a modal verb, so define stays in its base form.
If we had to define success, we would define it as feeling useful. The meaning applies only under that condition, not as a stated fact now.
Picture a family arguing about the word home. Someone says: If you asked ten people, each would define it differently. Nobody has asked yet. The sentence only paints an imagined result, so would define describes what might happen, not what has happened.
Or a teacher writing guidelines: We would define bullying as repeated unkind behavior. Here the definition belongs to a plan or proposal, so it stays hypothetical until adopted.
Watch the tense. She defines reports what someone actually does now. She would define only reports what she would say if the situation came up.