In one sentence
A claim against property by a contractor or supplier for unpaid work, which can jump ahead of a mortgage depending on state law and timing.
The longer version
Mechanic's liens secure payment for improvements and, in some states, relate back to when work began, threatening lien priority. Construction lenders manage this risk with lien waivers and title date-downs at each draw.
Mechanic's lien risk is why the construction draw package includes conditional and unconditional waivers and why title is re-checked before funds move.
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