The U.S. National Institute of Standards and Technology (NIST) defines Platform as a Service (PaaS) as follows:

The capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages, libraries, services, and tools supported by the provider. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, or storage, but has control over the deployed applications and possibly configuration settings for the application-hosting environment. (Mell & Grance, n.d.)

For more information on PaaS, you can read the 2018 IEEE 5G World Forum (5GWF) publication titled “A Vision for the Next Generation Platform-as-a-Service” by Rossem et al.