What is Guaranteed Maximum Price?

Legal Definition
A guaranteed maximum price (also known as GMP, not-to-exceed price, NTE, or NTX) contract is a cost-type contract (also known as an open-book contract) where the contractor is compensated for actual costs incurred plus a fixed fee subject to a ceiling price. The contractor is responsible for cost overruns, unless the GMP has been increased via formal change order (only as a result of additional scope from the client, not price overruns, errors, or omissions). Savings resulting from cost underruns are returned to the owner. This is different from a fixed-price contract (also known as stipulated price contract or lump-sum contract) where cost savings are typically retained by the contractor and essentially become additional profits.
-- Wikipedia
Legal Definition
The amount past what a client is made to pay a contractor.