A private network in Azure that lets Azure resources communicate with each other, with the internet, and with on-premises networks. It provides isolation, segmentation, and traffic control.
Fundamental in AZ-104 and SAA-C03 (the equivalent of AWS's VPC). AZ-900 covers it as a networking component. Key concepts: subnets, VNet peering, network security groups (NSGs). A VNet lives in a single region.
Azure Virtual Network (VNet) is a concept that can show up on the exam. Check the full guide to see it in context.