FaaS

Serverless computing, also known as function as a service (FaaS), is a cloud computing code execution model in which the cloud provider fully manages starting and stopping of a function's container platform as a service (PaaS) as necessary to serve requests, and requests are billed by an abstract measure of the resources required to satisfy the request, rather than per virtual machine, per hour.

Despite the name, it does not actually involve running code without servers. The name "serverless computing" is used because the business or person that owns the system does not have to purchase, rent or provision servers or virtual machines for the back-end code to run on.

Serverless code can be used in conjunction with code written in traditional server style, such as microservices. For example, part of a web application could be written as microservices and another part could be written as serverless code. Alternatively, an application could be written that uses no provisioned servers at all, being completely serverless.

Serverless code can either be triggered by specific events (such as user registration with Amazon Cognito), or be configured to run behind an API management platform in order to expose it as a REST API endpoint.

The first 'pay as you go' code execution platform where developers simply wrote code online including billing at a functional level, creation of virtual resources by the system as needed, exposure of functions as APIs was Zimki in 2006. AWS Lambda, introduced by Amazon in November 2014, was the first major provider considered to have a serverless offering. AWS Lambda initially launched with Node.js as the only runtime, but as of 2016^[update] it now officially supports Python, Java, C# and other languages such as Haskell can also be used by using Node.js as an invoker.

...
Wikipedia