A network segment within a VPC tied to a specific availability zone. Public subnets have a route to an Internet Gateway; private subnets have no direct internet access.
Fundamental for multi-tier architectures on CLF-C02 and SAA-C03. Typical pattern: web tier in public subnets, application and database tiers in private subnets. Each AZ has its own subnets for high availability.
Subnet (AWS) is a concept that can show up on the exam. Check the full guide to see it in context.