"Achilles heel" is a term commonly used to refer to someone's greatest weakness. However, there are various synonyms that can be used to describe this term. One such synonym is "weak spot," which refers to a vulnerable area or aspect of a person's character or abilities. Another synonym is "soft underbelly," which describes the part of someone's personality or conduct that is most susceptible to being wounded or exploited. "Vulnerability" is another synonym which can be used to describe someone's Achilles heel. It refers to a person's capacity to be emotionally or psychologically hurt, or to be taken advantage of. Overall, these synonyms are useful alternatives to describe the same concept.