Definition
A legal claim against property that secures a debt. Some are voluntary, like a mortgage on a house or a loan on a car; others arrive without consent, like tax liens, judgment liens, and contractor (mechanic's) liens. A lien generally must be paid off before the property can be sold with clear title.
Why it matters
Liens are why ownership can be conditional: the lienholder has rights to your property until the debt clears. Surprise liens surface at the worst moment, during a sale or refinance, and unpaid ones can escalate to foreclosure or seizure.