An Infrastructure as a Service, or IaaS for short, is a a cloud computing service, that enables users to instrument underlying physical infrastructure with much less effort and knowledge than it would be required when interfacing directly with it.
It allows the users of the service to interact with compute (or storage or database etc) resources through APIs, which makes it possible to encode whole Runtime Environments in Infrastructure as Code.
The primary IaaS providers are AWS, GCP and Azure, but there are many more.
The borderline between IaaS and PaaS (and even SaaS) is often blurry, as the offerings of most IaaS providers extend into all these realms.
