ABAP
In plain terms
SAP's proprietary language for building enterprise business applications.
Going deeper
ABAP runs inside the SAP ecosystem to customize and extend its massive enterprise software. It's specialized to SAP's data and transaction model.
Under the hood
Tightly coupled to SAP's database and application server; modern ABAP adds object orientation and cloud (RAP) development.
Examples
SAP ERP customization, business reports, enterprise workflows, S/4HANA.
Think of it like...
Like a house-specific language spoken only inside SAP's enormous corporate mansion.