How large a fall you can hold through without selling, which is a fact about you rather than about any stock. It has two halves people often confuse: the financial one, meaning whether a fall forces you to sell, and the emotional one, meaning whether it makes you want to. The second is the one that usually does the damage, and the only reliable way to learn yours is to notice how you actually behaved the last time your portfolio fell 30%.
See every term in the full PSE investing glossary.