Part 25 of the US Federal Air Regulations (FARs) is titled "AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES"
Part 25 of the US Federal Air Regulations deals with airworthiness standards for transport category airplanes.
It contains regulatory requirements pertaining to transport category aircraft certification in the USA.
Transport airplanes are airplanes for which a type certificate is applied for under part 21 in the transport category and that meet the transport category airworthiness requirements. Multi-engine airplanes with more than 19 seats or a maximum takeoff weight greater than 19,000 lb must be certificated in the transport category.
The full text of FAR 25 can be found online, as 14 CFR 25, as well as in printed form at most pilot shops.