The term "Achilles' heel" refers to a person's weakness or vulnerability, despite their overall strength. It originates from the Greek myth of Achilles, a great warrior whose only weak point was his heel. According to the legend, he was invulnerable except for this one area, which ultimately led to his downfall. In modern usage, "Achilles' heel" can describe any flaw or shortcoming that can lead to failure or defeat. It serves as a reminder that even the strongest individuals or systems can have critical weaknesses that may be exploited or lead to their undoing.