Architecture where each request is completely independent with no memory of previous requests. Like a vending machine that doesn't remember who used it before - every transaction is fresh.
A stateless web API treats each request separately, so you can send the same request to any server in a cluster and get the same result, making it easy to scale.
“Stateless” is an application design pattern, not a single cloud service. Most clouds support stateless architectures through load balancers, autoscaling, containers, and serverless platforms that can add/remove instances without losing user context (because state is stored externally).