48 CFR 16.201: 16.201 General.

(a) Fixed-price types of contracts provide for a firm price or, in appropriate cases, an adjustable price. Fixed-price contracts providing for an adjustable price may include a ceiling price, a target price (including target cost), or both. Unless otherwise specified in the contract, the ceiling price or target price is subject to adjustment only by operation of contract clauses providing for equitable adjustment or other revision of the contract price under stated circumstances. The contracting officer shall use firm-fixed-price or fixed-price with economic price adjustment contracts when acquiring commercial products and commercial services, except as provided in 12.207(b).

(b) Time-and-materials contracts and labor-hour contracts are not fixed-price contracts.