Definition
Unilateral mistake is when you alone are wrong about something important. Unlike mutual mistakes, it usually doesn't excuse the contract unless fraud is involved.
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Unilateral mistake (only you are wrong) generally doesn't excuse contract. Example: you think property is 2 acres; it's actually 1 acre. You're sole person mistaken. Generally, you're stuck with bad deal. Exception: if other party knew you were mistaken and didn't correct you, contract might be voidable (fraud by omission). Another exception: if mistake was obvious (property clearly smaller than you think) and other party said nothing, court might relieve you (depends on jurisdiction). Unilateral mistake is difficult to escape: parties generally bear risk of their own research. If you didn't do due diligence, mistake is usually your problem. Differs from mutual mistake: if both parties wrong, contract typically voidable; if only you're wrong, you're stuck. Understanding unilateral mistake: your responsibility to verify assumptions. Moral: verify everything; don't rely on other party's representations.
Pro Tip
Verify material facts yourself. Unilateral mistake is usually your problem.
Example
You believe property is zoned commercial; it's actually residential. You alone are mistaken (other party knew actual zoning). Generally, you're stuck unless you can prove fraud.