SAP Business Application Studio is a SAP Business Technology Platform (BTP) service that offers a modern development environment tailored for efficient development of business applications for the SAP Intelligent Enterprise. There is no software to download and install as it is a web browser based tool and all updates and new features are added by the cloud provider.

In SAP Business Application Studio, you are provided with one or more development spaces.
A development space - usually referred to as a dev space- is a development environment that contains all of the tools, capabilities, and resources needed for developing your application. Each type of application requires a different development environment and there are many dev spaces that are provided by SAP that already contain the tooling required to get started quickly. You can later add additional tools to supplement the development. For example, if you want to develop and consume SAP HANA calculation views from your application code, you could add the Graphical Calculation View Editor extension to your dev space.
SAP Business Application Studio is very similar to Microsoft Visual Studio. They are both powerful integrated development environments (IDE) and have a similar look and feel. It is possible to develop BTP applications using either. But SAP Business Application Studio is equipped with various tools and templates specially built for developing business applications in the SAP environment.
The SAP Business Application Studio includes graphical editors for the various development artifacts such as Fiori elements and calculation views, plus text editors for writing code. There is even a Cloud Foundry command line interface (CLI) to execute commands directly.
You can enrich the SAP Business Application Studio with tools that will help you develop faster and more efficiently, such as code assist, static code analysis, and code formatting.
SAP Business Application Studio is often referred to as BAS, but this is not an official SAP acronym.